diff --git a/pkg/dart2wasm/bin/dart2wasm.dart b/pkg/dart2wasm/bin/dart2wasm.dart
index 7119b7f21f11..7ca63921775c 100644
--- a/pkg/dart2wasm/bin/dart2wasm.dart
+++ b/pkg/dart2wasm/bin/dart2wasm.dart
@@ -28,16 +28,10 @@ final List<Option> options = [
   Flag(
       "lazy-constants", (o, value) => o.translatorOptions.lazyConstants = value,
       defaultsTo: _d.translatorOptions.lazyConstants),
-  Flag("local-nullability",
-      (o, value) => o.translatorOptions.localNullability = value,
-      defaultsTo: _d.translatorOptions.localNullability),
   Flag("name-section", (o, value) => o.translatorOptions.nameSection = value,
       defaultsTo: _d.translatorOptions.nameSection),
   Flag("nominal-types", (o, value) => o.translatorOptions.nominalTypes = value,
       defaultsTo: _d.translatorOptions.nominalTypes),
-  Flag("parameter-nullability",
-      (o, value) => o.translatorOptions.parameterNullability = value,
-      defaultsTo: _d.translatorOptions.parameterNullability),
   Flag("polymorphic-specialization",
       (o, value) => o.translatorOptions.polymorphicSpecialization = value,
       defaultsTo: _d.translatorOptions.polymorphicSpecialization),
diff --git a/pkg/dart2wasm/bin/run_wasm.js b/pkg/dart2wasm/bin/run_wasm.js
index a0673a0336e9..56d5248a3db8 100644
--- a/pkg/dart2wasm/bin/run_wasm.js
+++ b/pkg/dart2wasm/bin/run_wasm.js
@@ -6,7 +6,7 @@
 //
 // Run as follows:
 //
-// $> d8 --experimental-wasm-gc --wasm-gc-js-interop run_wasm.js -- <dart_module>.wasm [<ffi_module>.wasm]
+// $> d8 --experimental-wasm-gc --experimental-wasm-nn-locals --wasm-gc-js-interop run_wasm.js -- <dart_module>.wasm [<ffi_module>.wasm]
 //
 // If an FFI module is specified, it will be instantiated first, and its
 // exports will be supplied as imports to the Dart module under the 'ffi'
diff --git a/pkg/dart2wasm/dart2wasm.md b/pkg/dart2wasm/dart2wasm.md
index 6d3ab61ccd5f..74bfe1098e75 100644
--- a/pkg/dart2wasm/dart2wasm.md
+++ b/pkg/dart2wasm/dart2wasm.md
@@ -16,10 +16,8 @@ where *options* include:
 | `--`[`no-`]`import-shared-memory`       | no      | Import a shared memory buffer. If this is on, `--shared-memory-max-pages` must also be specified.
 | `--`[`no-`]`inlining`                   | no      | Inline small functions.
 | `--`[`no-`]`lazy-constants`             | no      | Instantiate constants lazily.
-| `--`[`no-`]`local-nullability`          | no      | Use non-nullable types for non-nullable locals and temporaries.
 | `--`[`no-`]`name-section`               | yes     | Emit Name Section with function names.
 | `--`[`no-`]`nominal-types`              | yes     | Emit nominal types.
-| `--`[`no-`]`parameter-nullability`      | yes     | Use non-nullable types for non-nullable parameters and return values.
 | `--`[`no-`]`polymorphic-specialization` | no      | Do virtual calls by switching on the class ID instead of using `call_indirect`.
 | `--`[`no-`]`print-kernel`               | no      | Print IR for each function before compiling it.
 | `--`[`no-`]`print-wasm`                 | no      | Print Wasm instructions of each compiled function.
@@ -30,7 +28,7 @@ where *options* include:
 
 The resulting `.wasm` file can be run with:
 
-`d8 --experimental-wasm-gc --wasm-gc-js-interop pkg/dart2wasm/bin/run_wasm.js -- `*outfile*`.wasm`
+`d8 --experimental-wasm-gc --experimental-wasm-nn-locals --wasm-gc-js-interop pkg/dart2wasm/bin/run_wasm.js -- `*outfile*`.wasm`
 
 ## Imports and exports
 
diff --git a/pkg/dart2wasm/lib/code_generator.dart b/pkg/dart2wasm/lib/code_generator.dart
index 66d7a680d0e3..39baec05d647 100644
--- a/pkg/dart2wasm/lib/code_generator.dart
+++ b/pkg/dart2wasm/lib/code_generator.dart
@@ -88,7 +88,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
   w.ValueType translateType(DartType type) => translator.translateType(type);
 
   w.Local addLocal(w.ValueType type) {
-    return function.addLocal(translator.typeForLocal(type));
+    return function.addLocal(type);
   }
 
   DartType dartTypeOf(Expression exp) {
@@ -205,7 +205,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
     void getThis() {
       w.Local thisLocal = paramLocals[0];
-      w.RefType structType = w.RefType.def(struct, nullable: true);
+      w.RefType structType = w.RefType.def(struct, nullable: false);
       b.local_get(thisLocal);
       translator.convertType(function, thisLocal.type, structType);
     }
@@ -338,16 +338,16 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         if (context.parent == null) break;
 
         b.struct_get(context.struct, context.parentFieldIndex);
-        if (options.localNullability) {
-          b.ref_as_non_null();
-        }
+        b.ref_as_non_null();
         context = context.parent!;
       }
       if (context.containsThis) {
-        thisLocal = addLocal(
-            context.struct.fields[context.thisFieldIndex].type.unpacked);
+        thisLocal = addLocal(context
+            .struct.fields[context.thisFieldIndex].type.unpacked
+            .withNullability(false));
         preciseThisLocal = thisLocal;
         b.struct_get(context.struct, context.thisFieldIndex);
+        b.ref_as_non_null();
         b.local_set(thisLocal!);
       }
     }
@@ -487,9 +487,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
   void visitRedirectingInitializer(RedirectingInitializer node) {
     Class cls = (node.parent as Constructor).enclosingClass;
     b.local_get(thisLocal!);
-    if (options.parameterNullability && thisLocal!.type.nullable) {
-      b.ref_as_non_null();
-    }
     for (TypeParameter typeParam in cls.typeParameters) {
       types.makeType(
           this, TypeParameterType(typeParam, Nullability.nonNullable));
@@ -507,9 +504,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       return;
     }
     b.local_get(thisLocal!);
-    if (options.parameterNullability && thisLocal!.type.nullable) {
-      b.ref_as_non_null();
-    }
     for (DartType typeArg in supertype!.typeArguments) {
       types.makeType(this, typeArg);
     }
@@ -882,7 +876,10 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         finalizer.mustHandleReturn = true;
       }
       if (returnType != voidMarker) {
-        returnValueLocal ??= addLocal(returnType);
+        // Since the flow of the return value through the returnValueLocal
+        // crosses control-flow constructs, the local needs to always have a
+        // defaultable type in order for the Wasm code to validate.
+        returnValueLocal ??= addLocal(returnType.withNullability(true));
         b.local_set(returnValueLocal!);
       }
       b.br(finalizers.last.label);
@@ -1001,8 +998,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         } else {
           wrap(exp, nonNullableType);
           b.local_get(switchValueNonNullableLocal);
-          translator.convertType(
-              function, switchValueNonNullableLocal.type, nonNullableType);
           compare();
           b.br_if(switchLabels[c]!);
         }
@@ -1095,15 +1090,17 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
   }
 
   w.ValueType visitThis(w.ValueType expectedType) {
-    w.ValueType thisType = thisLocal!.type.withNullability(false);
-    w.ValueType preciseThisType = preciseThisLocal!.type.withNullability(false);
+    w.ValueType thisType = thisLocal!.type;
+    w.ValueType preciseThisType = preciseThisLocal!.type;
+    assert(!thisType.nullable);
+    assert(!preciseThisType.nullable);
     if (!thisType.isSubtypeOf(expectedType) &&
         preciseThisType.isSubtypeOf(expectedType)) {
       b.local_get(preciseThisLocal!);
-      return preciseThisLocal!.type;
+      return preciseThisType;
     } else {
       b.local_get(thisLocal!);
-      return thisLocal!.type;
+      return thisType;
     }
   }
 
@@ -1122,9 +1119,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     b.local_get(temp);
     b.i32_const(info.classId);
     b.struct_set(info.struct, FieldIndex.classId);
-    if (options.parameterNullability && temp.type.nullable) {
-      b.ref_as_non_null();
-    }
     _visitArguments(node.arguments, node.targetReference, 1);
     call(node.targetReference);
     if (expectedType != voidMarker) {
@@ -1158,8 +1152,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         _lookupSuperTarget(node.interfaceTarget, setter: false).reference;
     w.BaseFunction targetFunction = translator.functions.getFunction(target);
     w.ValueType receiverType = targetFunction.type.inputs.first;
-    w.ValueType thisType = visitThis(receiverType);
-    translator.convertType(function, thisType, receiverType);
+    visitThis(receiverType);
     _visitArguments(node.arguments, target, 1);
     return call(target);
   }
@@ -1261,14 +1254,12 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         b.local_get(leftLocal);
         if (leftNullable) {
           b.br_on_null(operandNull!);
-        } else if (leftLocal.type.nullable) {
-          b.ref_as_non_null();
         }
       }
 
       void right([_]) {
         b.local_get(rightLocal);
-        if (rightLocal.type.nullable) {
+        if (rightNullable) {
           b.ref_as_non_null();
         }
       }
@@ -1339,10 +1330,8 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
     // Receiver is already on stack.
     w.Local receiverVar = addLocal(selector.signature.inputs.first);
+    assert(!receiverVar.type.nullable);
     b.local_tee(receiverVar);
-    if (options.parameterNullability && receiverVar.type.nullable) {
-      b.ref_as_non_null();
-    }
     pushArguments(selector.signature);
 
     if (options.polymorphicSpecialization) {
@@ -1610,10 +1599,8 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     // Evaluate receiver
     wrap(node.receiver, selector.signature.inputs.first);
     w.Local receiverVar = addLocal(selector.signature.inputs.first);
+    assert(!receiverVar.type.nullable);
     b.local_tee(receiverVar);
-    if (options.parameterNullability && receiverVar.type.nullable) {
-      b.ref_as_non_null();
-    }
 
     // Dispatch table call
     b.comment("Dynamic get of '${selector.name}'");
@@ -1714,7 +1701,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       if (preserved) {
         temp = addLocal(paramType);
         b.local_tee(temp);
-        translator.convertType(function, temp.type, paramType);
       }
       call(target.reference);
     }
@@ -1779,9 +1765,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     Context? context = closures.contexts[functionNode]?.parent;
     if (context != null) {
       b.local_get(context.currentLocal);
-      if (context.currentLocal.type.nullable) {
-        b.ref_as_non_null();
-      }
+      assert(!context.currentLocal.type.nullable);
     } else {
       b.global_get(translator.globals.dummyGlobal); // Dummy context
     }
@@ -1908,7 +1892,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
           signature.inputs[signatureOffset + paramInfo.nameIndex[name]!];
       if (namedLocal != null) {
         b.local_get(namedLocal);
-        translator.convertType(function, namedLocal.type, type);
       } else {
         translator.constants
             .instantiateConstant(function, b, paramInfo.named[name]!, type);
@@ -2029,9 +2012,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
           b.array_set(arrayType);
         }
         b.local_get(arrayLocal);
-        if (arrayLocal.type.nullable) {
-          b.ref_as_non_null();
-        }
       }
     } else {
       for (int i = 0; i < length; i++) {
@@ -2072,7 +2052,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     b.local_set(mapLocal);
     for (MapLiteralEntry entry in node.entries) {
       b.local_get(mapLocal);
-      translator.convertType(function, mapLocal.type, putReceiverType);
       wrap(entry.key, putKeyType);
       wrap(entry.value, putValueType);
       b.call(mapPut);
@@ -2100,7 +2079,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
     b.local_set(setLocal);
     for (Expression element in node.expressions) {
       b.local_get(setLocal);
-      translator.convertType(function, setLocal.type, addReceiverType);
       wrap(element, addKeyType);
       b.call(setAdd);
       b.drop();
@@ -2161,8 +2139,7 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       Class cls = parameter.parent as Class;
       ClassInfo info = translator.classInfo[cls]!;
       int fieldIndex = translator.typeParameterIndex[parameter]!;
-      w.ValueType thisType = visitThis(info.nonNullableType);
-      translator.convertType(function, thisType, info.nonNullableType);
+      visitThis(info.nonNullableType);
       b.struct_get(info.struct, fieldIndex);
       resultType = info.struct.fields[fieldIndex].type.unpacked;
     }
diff --git a/pkg/dart2wasm/lib/constants.dart b/pkg/dart2wasm/lib/constants.dart
index 70df2ad1206c..b83d6e747d7c 100644
--- a/pkg/dart2wasm/lib/constants.dart
+++ b/pkg/dart2wasm/lib/constants.dart
@@ -212,8 +212,8 @@ class Constants {
 
     w.Local offset = function.locals[0];
     w.Local length = function.locals[1];
-    w.Local array = function.addLocal(
-        translator.typeForLocal(w.RefType.def(arrayType, nullable: false)));
+    w.Local array =
+        function.addLocal(w.RefType.def(arrayType, nullable: false));
     w.Local index = function.addLocal(w.NumType.i32);
 
     w.Instructions b = function.body;
@@ -422,12 +422,11 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
       w.FunctionType ftype = translator.functionType(const [], [type]);
       w.DefinedFunction function = m.addFunction(ftype, "$constant");
       generator(function, function.body);
-      w.Local temp = function.addLocal(translator.typeForLocal(type));
+      w.Local temp = function.addLocal(type);
       w.Instructions b2 = function.body;
       b2.local_tee(temp);
       b2.global_set(global);
       b2.local_get(temp);
-      translator.convertType(function, temp.type, type);
       b2.end();
 
       return ConstantInfo(constant, global, function);
@@ -579,8 +578,7 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
       b.i64_const(length);
       if (lazyConstants) {
         // Allocate array and set each entry to the corresponding sub-constant.
-        w.Local arrayLocal = function!.addLocal(
-            refType.withNullability(!translator.options.localNullability));
+        w.Local arrayLocal = function!.addLocal(refType.withNullability(false));
         b.i32_const(length);
         translator.array_new_default(b, arrayType);
         b.local_set(arrayLocal);
@@ -592,9 +590,6 @@ class ConstantCreator extends ConstantVisitor<ConstantInfo?> {
           b.array_set(arrayType);
         }
         b.local_get(arrayLocal);
-        if (arrayLocal.type.nullable) {
-          b.ref_as_non_null();
-        }
       } else {
         // Push all sub-constants on the stack and initialize array from them.
         for (int i = 0; i < length; i++) {
diff --git a/pkg/dart2wasm/lib/dispatch_table.dart b/pkg/dart2wasm/lib/dispatch_table.dart
index a283258c6557..5682edac0bb9 100644
--- a/pkg/dart2wasm/lib/dispatch_table.dart
+++ b/pkg/dart2wasm/lib/dispatch_table.dart
@@ -121,7 +121,7 @@ class SelectorInfo {
     });
 
     List<w.ValueType> typeParameters = List.filled(paramInfo.typeParamCount,
-        translator.classInfo[translator.typeClass]!.nullableType);
+        translator.classInfo[translator.typeClass]!.nonNullableType);
     List<w.ValueType> inputs = List.generate(
         inputSets.length,
         (i) => translator.typeForInfo(
diff --git a/pkg/dart2wasm/lib/functions.dart b/pkg/dart2wasm/lib/functions.dart
index 6c190c7a4207..3f938f3c4e67 100644
--- a/pkg/dart2wasm/lib/functions.dart
+++ b/pkg/dart2wasm/lib/functions.dart
@@ -195,7 +195,7 @@ class FunctionCollector extends MemberVisitor1<w.FunctionType, Reference> {
     }
 
     List<w.ValueType> typeParameters = List.filled(typeParamCount,
-        translator.classInfo[translator.typeClass]!.nullableType);
+        translator.classInfo[translator.typeClass]!.nonNullableType);
 
     // The JS embedder will not accept Wasm struct types as parameter or return
     // types for functions called from JS. We need to use eqref instead.
diff --git a/pkg/dart2wasm/lib/intrinsics.dart b/pkg/dart2wasm/lib/intrinsics.dart
index 780f9270940b..65dd8b75070d 100644
--- a/pkg/dart2wasm/lib/intrinsics.dart
+++ b/pkg/dart2wasm/lib/intrinsics.dart
@@ -811,7 +811,7 @@ class Intrinsifier {
           codeGen.wrap(
               node.arguments.positional.single, receiverInfo.nonNullableType);
           w.Local receiverLocal =
-              codeGen.function.addLocal(receiverInfo.nullableType);
+              codeGen.function.addLocal(receiverInfo.nonNullableType);
           b.local_tee(receiverLocal);
           // We ignore the type argument and just update the classID of the
           // receiver.
@@ -823,7 +823,6 @@ class Intrinsifier {
           b.i32_const(fixedLengthListInfo.classId);
           b.struct_set(topInfo.struct, FieldIndex.classId);
           b.local_get(receiverLocal);
-          b.ref_as_non_null();
           return fixedLengthListInfo.nonNullableType;
       }
     }
diff --git a/pkg/dart2wasm/lib/translator.dart b/pkg/dart2wasm/lib/translator.dart
index 676d4bb3d9d2..f54810a488c0 100644
--- a/pkg/dart2wasm/lib/translator.dart
+++ b/pkg/dart2wasm/lib/translator.dart
@@ -34,10 +34,8 @@ class TranslatorOptions {
   bool inlining = false;
   int inliningLimit = 3;
   bool lazyConstants = false;
-  bool localNullability = false;
   bool nameSection = true;
   bool nominalTypes = true;
-  bool parameterNullability = true;
   bool polymorphicSpecialization = false;
   bool printKernel = false;
   bool printWasm = false;
@@ -508,8 +506,7 @@ class Translator {
         return w.NumType.i32;
       }
     }
-    return w.RefType.def(info.repr.struct,
-        nullable: !options.parameterNullability || nullable);
+    return w.RefType.def(info.repr.struct, nullable: nullable);
   }
 
   w.StorageType translateStorageType(DartType type) {
@@ -558,7 +555,7 @@ class Translator {
           : type.bound);
     }
     if (type is FutureOrType) {
-      return topInfo.nullableType;
+      return topInfo.typeWithNullability(type.isPotentiallyNullable);
     }
     if (type is FunctionType) {
       if (type.requiredParameterCount != type.positionalParameters.length ||
@@ -566,8 +563,7 @@ class Translator {
         throw "Function types with optional parameters not supported: $type";
       }
       return w.RefType.def(closureStructType(type.requiredParameterCount),
-          nullable:
-              !options.parameterNullability || type.isPotentiallyNullable);
+          nullable: type.isPotentiallyNullable);
     }
     throw "Unsupported type ${type.runtimeType}";
   }
@@ -662,10 +658,6 @@ class Translator {
     });
   }
 
-  w.ValueType typeForLocal(w.ValueType type) {
-    return options.localNullability ? type : type.withNullability(true);
-  }
-
   w.ValueType outputOrVoid(List<w.ValueType> outputs) {
     return outputs.isEmpty ? voidMarker : outputs.single;
   }
diff --git a/pkg/dart2wasm/lib/types.dart b/pkg/dart2wasm/lib/types.dart
index 1011b26a4da2..b60f7a52f4ec 100644
--- a/pkg/dart2wasm/lib/types.dart
+++ b/pkg/dart2wasm/lib/types.dart
@@ -74,8 +74,6 @@ class Types {
   Iterable<Class> _getConcreteSubtypes(Class cls) =>
       translator.subtypes.getSubtypesOf(cls).where((c) => !c.isAbstract);
 
-  w.ValueType get nullableTypeType => typeClassInfo.nullableType;
-
   w.ValueType get nonNullableTypeType => typeClassInfo.nonNullableType;
 
   InterfaceType get namedParameterType =>
diff --git a/pkg/test_runner/lib/src/compiler_configuration.dart b/pkg/test_runner/lib/src/compiler_configuration.dart
index c5e47c136a22..190e81d37b83 100644
--- a/pkg/test_runner/lib/src/compiler_configuration.dart
+++ b/pkg/test_runner/lib/src/compiler_configuration.dart
@@ -573,6 +573,7 @@ class Dart2WasmCompilerConfiguration extends CompilerConfiguration {
       CommandArtifact? artifact) {
     return [
       '--experimental-wasm-gc',
+      '--experimental-wasm-nn-locals',
       '--wasm-gc-js-interop',
       'pkg/dart2wasm/bin/run_wasm.js',
       '--',
diff --git a/pkg/wasm_builder/lib/src/instructions.dart b/pkg/wasm_builder/lib/src/instructions.dart
index 64fdd47e50f6..7711a0923bdd 100644
--- a/pkg/wasm_builder/lib/src/instructions.dart
+++ b/pkg/wasm_builder/lib/src/instructions.dart
@@ -28,6 +28,7 @@ abstract class Label {
   late final int depth;
   late final int baseStackHeight;
   late final bool reachable;
+  late final int localInitializationStackHeight;
 
   Label._(this.inputs, this.outputs);
 
@@ -46,6 +47,7 @@ class Expression extends Label {
     depth = 0;
     baseStackHeight = 0;
     reachable = true;
+    localInitializationStackHeight = 0;
   }
 
   List<ValueType> get targetTypes => outputs;
@@ -95,7 +97,7 @@ class Instructions with SerializerMixin {
   final Module module;
 
   /// Locals declared in this body, including parameters.
-  final List<Local> locals;
+  final List<Local> locals = [];
 
   /// Is this the initializer of a global variable?
   final bool isGlobalInitializer;
@@ -129,9 +131,15 @@ class Instructions with SerializerMixin {
   final List<ValueType> _stackTypes = [];
   bool _reachable = true;
 
+  /// Whether each local is currently definitely initialized.
+  final List<bool> _localInitialized = [];
+
+  /// Stack of currently initialized non-defaultable locals.
+  final List<int> _localInitializationStack = [];
+
   /// Create a new instruction sequence.
   Instructions(this.module, List<ValueType> outputs,
-      {this.locals = const [], this.isGlobalInitializer = false}) {
+      {this.isGlobalInitializer = false}) {
     _labelStack.add(Expression(const [], outputs));
   }
 
@@ -144,6 +152,32 @@ class Instructions with SerializerMixin {
   /// Textual trace of the instructions.
   String get trace => _traceLines.join();
 
+  Local addLocal(ValueType type, {required bool isParameter}) {
+    Local local = Local(locals.length, type);
+    locals.add(local);
+    _localInitialized.add(isParameter || type.defaultable);
+    return local;
+  }
+
+  bool _initializeLocal(Local local) {
+    if (!_localInitialized[local.index]) {
+      _localInitialized[local.index] = true;
+      _localInitializationStack.add(local.index);
+    }
+    return true;
+  }
+
+  bool _localIsInitialized(Local local) {
+    return _localInitialized[local.index];
+  }
+
+  void _resetLocalInitialization(Label label) {
+    while (_localInitializationStack.length >
+        label.localInitializationStackHeight) {
+      _localInitialized[_localInitializationStack.removeLast()] = false;
+    }
+  }
+
   bool _debugTrace(List<Object>? trace,
       {required bool reachableAfter,
       int indentBefore = 0,
@@ -289,6 +323,7 @@ class Instructions with SerializerMixin {
       _stackTypes.length = label.baseStackHeight;
       _stackTypes.addAll(outputs);
     }
+    _resetLocalInitialization(label);
     return _debugTrace([if (label.hasOrdinal) "$label:", ...trace],
         reachableAfter: reachableAfter,
         indentBefore: -1,
@@ -324,6 +359,7 @@ class Instructions with SerializerMixin {
     label.depth = _labelStack.length;
     label.baseStackHeight = _stackTypes.length - label.inputs.length;
     label.reachable = reachable;
+    label.localInitializationStackHeight = _localInitializationStack.length;
     _labelStack.add(label);
     assert(_verifyStartOfBlock(label, trace: trace));
     writeByte(encoding);
@@ -545,6 +581,8 @@ class Instructions with SerializerMixin {
   void local_get(Local local) {
     assert(locals[local.index] == local);
     assert(_verifyTypes(const [], [local.type], trace: ['local.get', local]));
+    assert(_localIsInitialized(local) ||
+        _reportError("Uninitialized local with non-defaultable type"));
     writeByte(0x20);
     writeUnsigned(local.index);
   }
@@ -553,6 +591,7 @@ class Instructions with SerializerMixin {
   void local_set(Local local) {
     assert(locals[local.index] == local);
     assert(_verifyTypes([local.type], const [], trace: ['local.set', local]));
+    assert(_initializeLocal(local));
     writeByte(0x21);
     writeUnsigned(local.index);
   }
@@ -562,6 +601,7 @@ class Instructions with SerializerMixin {
     assert(locals[local.index] == local);
     assert(
         _verifyTypes([local.type], [local.type], trace: ['local.tee', local]));
+    assert(_initializeLocal(local));
     writeByte(0x22);
     writeUnsigned(local.index);
   }
diff --git a/pkg/wasm_builder/lib/src/module.dart b/pkg/wasm_builder/lib/src/module.dart
index 3c0a55a020d7..05629002b5c6 100644
--- a/pkg/wasm_builder/lib/src/module.dart
+++ b/pkg/wasm_builder/lib/src/module.dart
@@ -354,25 +354,21 @@ class DefinedFunction extends BaseFunction
     with SerializerMixin
     implements Serializable {
   /// All local variables defined in the function, including its inputs.
-  final List<Local> locals = [];
+  List<Local> get locals => body.locals;
 
   /// The body of the function.
   late final Instructions body;
 
   DefinedFunction(Module module, super.index, super.type,
       [super.functionName]) {
+    body = Instructions(module, type.outputs);
     for (ValueType paramType in type.inputs) {
-      addLocal(paramType);
+      body.addLocal(paramType, isParameter: true);
     }
-    body = Instructions(module, type.outputs, locals: locals);
   }
 
   /// Add a local variable to the function.
-  Local addLocal(ValueType type) {
-    Local local = Local(locals.length, type);
-    locals.add(local);
-    return local;
-  }
+  Local addLocal(ValueType type) => body.addLocal(type, isParameter: false);
 
   @override
   void serialize(Serializer s) {