[dart2wasm] Fix handling of null values in some non-nullable contexts

Change-Id: Id25f4899d0898686ee30bf05f7e57a9ef84fb818
Cq-Include-Trybots: luci.dart.try:dart2wasm-linux-x64-d8-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/252740
Reviewed-by: Joshua Litt <joshualitt@google.com>

pkg/dart2wasm/lib/code_generator.dart

@@ -1857,17 +1857,20 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
   @override
   w.ValueType visitNullCheck(NullCheck node, w.ValueType expectedType) {
-    w.ValueType operandType =
-        translator.translateType(dartTypeOf(node.operand));
+    return _nullCheck(node.operand, translator.throwNullCheckError);
+  }
+
+  w.ValueType _nullCheck(Expression operand, Procedure errorProcedure) {
+    w.ValueType operandType = translator.translateType(dartTypeOf(operand));
     w.ValueType nonNullOperandType = operandType.withNullability(false);
     w.Label nullCheckBlock = b.block(const [], [nonNullOperandType]);
-    wrap(node.operand, operandType);
+    wrap(operand, operandType);
 
     // We lower a null check to a br_on_non_null, throwing a [TypeError] in the
     // null case.
     b.br_on_non_null(nullCheckBlock);
     call(translator.stackTraceCurrent.reference);
-    call(translator.throwNullCheckError.reference);
+    call(errorProcedure.reference);
     b.unreachable();
     b.end();
     return nonNullOperandType;
@@ -1923,7 +1926,11 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
   @override
   w.ValueType visitThrow(Throw node, w.ValueType expectedType) {
-    wrap(node.expression, translator.topInfo.nonNullableType);
+    if (dartTypeOf(node.expression).isPotentiallyNullable) {
+      _nullCheck(node.expression, translator.throwThrowNullError);
+    } else {
+      wrap(node.expression, translator.topInfo.nonNullableType);
+    }
     call(translator.stackTraceCurrent.reference);
 
     // At this point, we have the exception and the current stack trace on the

pkg/dart2wasm/lib/intrinsics.dart

@@ -714,10 +714,10 @@ class Intrinsifier {
       switch (name) {
         case "unsafeCast":
         case "unsafeCastOpaque":
-          w.ValueType targetType =
-              translator.translateType(node.arguments.types.single);
           Expression operand = node.arguments.positional.single;
-          return codeGen.wrap(operand, targetType);
+          // Just evaluate the operand and let the context convert it to the
+          // expected type.
+          return codeGen.wrap(operand, typeOfExp(operand));
         case "_nativeEffect":
           // Ignore argument
           return translator.voidMarker;

pkg/dart2wasm/lib/translator.dart

@@ -119,6 +119,7 @@ class Translator {
   late final Procedure stringEquals;
   late final Procedure stringInterpolate;
   late final Procedure throwNullCheckError;
+  late final Procedure throwThrowNullError;
   late final Procedure throwAsCheckError;
   late final Procedure throwWasmRefError;
   late final Procedure mapFactory;
@@ -250,6 +251,8 @@ class Translator {
         .firstWhere((p) => p.name.text == "_interpolate");
     throwNullCheckError = typeErrorClass.procedures
         .firstWhere((p) => p.name.text == "_throwNullCheckError");
+    throwThrowNullError = typeErrorClass.procedures
+        .firstWhere((p) => p.name.text == "_throwThrowNullError");
     throwAsCheckError = typeErrorClass.procedures
         .firstWhere((p) => p.name.text == "_throwAsCheckError");
     throwWasmRefError = typeErrorClass.procedures

sdk/lib/_internal/wasm/lib/errors_patch.dart

@@ -41,7 +41,7 @@ class _Error extends Error {
 }
 
 class _TypeError extends _Error implements TypeError {
-  _TypeError(String message) : super('TypeError: $message');
+  _TypeError(String message) : super(message);
 
   factory _TypeError.fromMessageAndStackTrace(
       String message, StackTrace stackTrace) {
@@ -57,11 +57,19 @@ class _TypeError extends _Error implements TypeError {
     return _throwObjectWithStackTrace(typeError, stackTrace);
   }
 
+  @pragma("wasm:entry-point")
+  static Never _throwThrowNullError(StackTrace stackTrace) {
+    final typeError =
+        _TypeError.fromMessageAndStackTrace("Throw of null", stackTrace);
+    return _throwObjectWithStackTrace(typeError, stackTrace);
+  }
+
   @pragma("wasm:entry-point")
   static Never _throwAsCheckError(
       Object? operand, Type? type, StackTrace stackTrace) {
     final typeError = _TypeError.fromMessageAndStackTrace(
-        "Type '${operand.runtimeType}' is not a subtype of type '$type' in type cast",
+        "Type '${operand.runtimeType}' is not a subtype of type '$type'"
+        " in type cast",
         stackTrace);
     return _throwObjectWithStackTrace(typeError, stackTrace);
   }