Revert "[kernel] Ensure that visitors don't implicitly returns null"

This reverts commit ce81216.

Reason for revert: Flutter dependency

Original change's description:
> [kernel] Ensure that visitors don't implicitly returns `null`
>
> This is in preparation to migrate package:kernel to null safety.
> For the visitor interfaces to support non-nullable return types, the
> implementations must avoid using `null` as return value in its base case.
>
> TEST=Refactoring
>
> Change-Id: Ie5e4153f8d3779d94957bb13b3d2d2a942040ff2
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/179760
> Commit-Queue: Johnni Winther <johnniwinther@google.com>
> Reviewed-by: Jens Johansen <jensj@google.com>

TBR=jensj@google.com,johnniwinther@google.com

Change-Id: I61b838d3371e6b1de2427716d056324c120be499
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183689
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/_js_interop_checks/lib/js_interop_checks.dart

@@ -23,7 +23,7 @@ import 'package:_fe_analyzer_shared/src/messages/codes.dart'
 
 import 'src/js_interop.dart';
 
-class JsInteropChecks extends RecursiveVisitor {
+class JsInteropChecks extends RecursiveVisitor<void> {
   final CoreTypes _coreTypes;
   final DiagnosticReporter<Message, LocatedMessage> _diagnosticsReporter;
   final Map<String, Class> _nativeClasses;

pkg/compiler/lib/src/inferrer/builder_kernel.dart

@@ -37,8 +37,7 @@ import 'type_system.dart';
 /// Calling [run] will start the work of visiting the body of the code to
 /// construct a set of inference-nodes that abstractly represent what the code
 /// is doing.
-class KernelTypeGraphBuilder extends ir.Visitor<TypeInformation>
-    with ir.VisitorNullMixin<TypeInformation> {
+class KernelTypeGraphBuilder extends ir.Visitor<TypeInformation> {
   final CompilerOptions _options;
   final JsClosedWorld _closedWorld;
   final InferrerEngine _inferrer;

pkg/compiler/lib/src/ir/debug.dart

@@ -11,8 +11,7 @@ import 'package:kernel/visitor.dart';
 
 import '../util/util.dart' show Indentation, Tagging;
 
-class DebugPrinter extends Visitor<void>
-    with Indentation, Tagging<Node>, VisitorVoidMixin {
+class DebugPrinter extends Visitor with Indentation, Tagging<Node> {
   @override
   StringBuffer sb = new StringBuffer();
 

pkg/compiler/lib/src/ir/scope_visitor.dart

@@ -14,7 +14,7 @@ import 'scope.dart';
 /// a [VariableScopeModel] that can respond to queries about how a particular
 /// variable is being used at any point in the code.
 class ScopeModelBuilder extends ir.Visitor<EvaluationComplexity>
-    with VariableCollectorMixin, ir.VisitorNullMixin<EvaluationComplexity> {
+    with VariableCollectorMixin {
   final Dart2jsConstantEvaluator _constantEvaluator;
   ir.StaticTypeContext _staticTypeContext;
 

pkg/compiler/lib/src/ir/static_type_base.dart

@@ -57,8 +57,7 @@ class ExactInterfaceType extends ir.InterfaceType {
 /// expression kind. For instance method invocations whose static type depend
 /// on the static types of the receiver and type arguments and the signature
 /// of the targeted procedure.
-abstract class StaticTypeBase extends ir.Visitor<ir.DartType>
-    with ir.VisitorNullMixin<ir.DartType> {
+abstract class StaticTypeBase extends ir.Visitor<ir.DartType> {
   final ir.TypeEnvironment _typeEnvironment;
 
   StaticTypeBase(this._typeEnvironment);

pkg/compiler/lib/src/ir/visitors.dart

@@ -35,9 +35,6 @@ class Stringifier extends ir.ExpressionVisitor<String> {
     }
     return null;
   }
-
-  @override
-  String defaultExpression(ir.Expression node) => null;
 }
 
 /// Visitor that converts kernel dart types into [DartType].
@@ -185,11 +182,6 @@ class DartTypeConverter extends ir.DartTypeVisitor<DartType> {
   DartType visitNullType(ir.NullType node) {
     return elementMap.commonElements.nullType;
   }
-
-  @override
-  DartType defaultDartType(ir.DartType node) {
-    throw UnsupportedError('Unsupported type $node (${node.runtimeType})');
-  }
 }
 
 class ConstantValuefier extends ir.ComputeOnceConstantVisitor<ConstantValue> {

pkg/compiler/lib/src/js_model/locals.dart

@@ -292,7 +292,7 @@ class KernelToLocalsMapImpl implements KernelToLocalsMap {
   }
 }
 
-class JumpVisitor extends ir.Visitor<void> with ir.VisitorVoidMixin {
+class JumpVisitor extends ir.Visitor {
   int jumpIndex = 0;
   int labelIndex = 0;
   final MemberEntity member;

pkg/compiler/lib/src/serialization/node_indexer.dart

@@ -6,8 +6,7 @@ part of 'serialization.dart';
 
 /// Visitor that ascribes an index to all [ir.TreeNode]s that potentially
 /// needed for serialization and deserialization.
-class _TreeNodeIndexerVisitor extends ir.Visitor<void>
-    with ir.VisitorVoidMixin {
+class _TreeNodeIndexerVisitor extends ir.Visitor<void> {
   int _currentIndex = 0;
   final Map<int, ir.TreeNode> _indexToNodeMap;
   final Map<ir.TreeNode, int> _nodeToIndexMap;

pkg/compiler/lib/src/ssa/builder_kernel.dart

@@ -84,7 +84,7 @@ class StackFrame {
       this.staticTypeProvider);
 }
 
-class KernelSsaGraphBuilder extends ir.Visitor<void> with ir.VisitorVoidMixin {
+class KernelSsaGraphBuilder extends ir.Visitor {
   /// Holds the resulting SSA graph.
   final HGraph graph = new HGraph();
 
@@ -6560,8 +6560,7 @@ class KernelTypeBuilder extends TypeBuilder {
   }
 }
 
-class _ErroneousInitializerVisitor extends ir.Visitor<bool>
-    with ir.VisitorDefaultValueMixin<bool> {
+class _ErroneousInitializerVisitor extends ir.Visitor<bool> {
   _ErroneousInitializerVisitor();
 
   // TODO(30809): Use const constructor.
@@ -6581,7 +6580,7 @@ class _ErroneousInitializerVisitor extends ir.Visitor<bool>
 
   // Expressions: Does the expression always throw?
   @override
-  bool get defaultValue => false;
+  bool defaultExpression(ir.Expression node) => false;
 
   @override
   bool visitThrow(ir.Throw node) => true;
@@ -6840,7 +6839,7 @@ class InlineDataCache {
   }
 }
 
-class InlineWeeder extends ir.Visitor<void> with ir.VisitorVoidMixin {
+class InlineWeeder extends ir.Visitor {
   // Invariant: *INSIDE_LOOP* > *OUTSIDE_LOOP*
   static const INLINING_NODES_OUTSIDE_LOOP = 15;
   static const INLINING_NODES_OUTSIDE_LOOP_ARG_FACTOR = 3;
@@ -7398,8 +7397,7 @@ class InlineWeeder extends ir.Visitor<void> with ir.VisitorVoidMixin {
 
 /// Visitor to detect environment-rewriting that prevents inlining
 /// (e.g. closures).
-class InlineWeederBodyClosure extends ir.Visitor<void>
-    with ir.VisitorVoidMixin {
+class InlineWeederBodyClosure extends ir.Visitor<void> {
   bool tooDifficult = false;
 
   InlineWeederBodyClosure();

pkg/compiler/lib/src/ssa/kernel_string_builder.dart

@@ -10,7 +10,7 @@ import 'builder_kernel.dart';
 import 'nodes.dart';
 
 /// Visits and concatenates the expressions in a string concatenation.
-class KernelStringBuilder extends ir.Visitor<void> with ir.VisitorVoidMixin {
+class KernelStringBuilder extends ir.Visitor {
   final KernelSsaGraphBuilder builder;
 
   /// The string value generated so far.

pkg/compiler/lib/src/ssa/loop_handler.dart

@@ -329,10 +329,9 @@ class KernelLoopHandler extends LoopHandler {
   int loopKind(ir.TreeNode node) => node.accept(new _KernelLoopTypeVisitor());
 }
 
-class _KernelLoopTypeVisitor extends ir.Visitor<int>
-    with ir.VisitorDefaultValueMixin<int> {
+class _KernelLoopTypeVisitor extends ir.Visitor<int> {
   @override
-  int get defaultValue => HLoopBlockInformation.NOT_A_LOOP;
+  int defaultNode(ir.Node node) => HLoopBlockInformation.NOT_A_LOOP;
 
   @override
   int visitWhileStatement(ir.WhileStatement node) =>

pkg/compiler/lib/src/ssa/switch_continue_analysis.dart

@@ -7,8 +7,7 @@ import 'package:kernel/ast.dart' as ir;
 /// Helper class that traverses a kernel AST subtree to see if it has any
 /// continue statements in the body of any switch cases (having continue
 /// statements results in a more complex generated code).
-class SwitchContinueAnalysis extends ir.Visitor<bool>
-    with ir.VisitorDefaultValueMixin<bool> {
+class SwitchContinueAnalysis extends ir.Visitor<bool> {
   SwitchContinueAnalysis._();
 
   static bool containsContinue(ir.Statement switchCaseBody) {
@@ -128,5 +127,5 @@ class SwitchContinueAnalysis extends ir.Visitor<bool>
   }
 
   @override
-  bool get defaultValue => false;
+  bool defaultNode(ir.Node node) => false;
 }

pkg/dev_compiler/lib/src/kernel/expression_compiler.dart

@@ -36,9 +36,7 @@ import 'package:kernel/ast.dart'
         TreeNode,
         TypeParameter,
         VariableDeclaration,
-        Visitor,
-        VisitorNullMixin,
-        VisitorVoidMixin;
+        Visitor;
 
 DiagnosticMessage _createInternalError(Uri uri, int line, int col, String msg) {
   return Message(Code<String>('Expression Compiler Internal error'),
@@ -86,7 +84,7 @@ class DartScope {
 /// - locals
 /// - formals
 /// - captured variables (for closures)
-class DartScopeBuilder extends Visitor<void> with VisitorVoidMixin {
+class DartScopeBuilder extends Visitor<void> {
   final Component _component;
   final int _line;
   final int _column;
@@ -198,7 +196,7 @@ class DartScopeBuilder extends Visitor<void> with VisitorVoidMixin {
 /// that do not have .fileEndOffset field.
 ///
 /// For example - [Block]
-class FileEndOffsetCalculator extends Visitor<int> with VisitorNullMixin<int> {
+class FileEndOffsetCalculator extends Visitor<int> {
   static const int noOffset = -1;
 
   final int _startOffset;
@@ -268,7 +266,7 @@ class FileEndOffsetCalculator extends Visitor<int> with VisitorNullMixin<int> {
 /// in the JavaScript scope, so we need to redefine them.
 ///
 /// See [_addSymbolDefinitions]
-class PrivateFieldsVisitor extends Visitor<void> with VisitorVoidMixin {
+class PrivateFieldsVisitor extends Visitor<void> {
   final Map<String, Library> privateFields = {};
 
   @override

pkg/dev_compiler/lib/src/kernel/kernel_helpers.dart

@@ -332,9 +332,6 @@ class LabelContinueFinder extends StatementVisitor<void> {
     visit(node.body);
     visit(node.finalizer);
   }
-
-  @override
-  void defaultStatement(Statement node) {}
 }
 
 /// Ensures that all of the known DartType implementors are handled.

pkg/dev_compiler/lib/src/kernel/nullable_inference.dart

@@ -364,7 +364,7 @@ class NullableInference extends ExpressionVisitor<bool> {
 /// variables that have already been determined to be nullable.
 ///
 // TODO(jmesserly): Introduce flow analysis.
-class _NullableVariableInference extends RecursiveVisitor {
+class _NullableVariableInference extends RecursiveVisitor<void> {
   NullableInference _nullInference;
 
   /// Variables that are currently believed to be not-null.

pkg/dev_compiler/lib/src/kernel/target.dart

@@ -245,7 +245,7 @@ class DevCompilerTarget extends Target {
 /// members can be eliminated, and adjusts the flags to remove those checks.
 ///
 /// See [_CovarianceTransformer.transform].
-class _CovarianceTransformer extends RecursiveVisitor {
+class _CovarianceTransformer extends RecursiveVisitor<void> {
   /// The set of private instance members in [_library] that (potentially) need
   /// covariance checks.
   ///

pkg/dev_compiler/test/expression_compiler/scope_offset_test.dart

@@ -34,7 +34,7 @@ void main(List<String> args) {
   });
 }
 
-class ScopeOffsetValidator extends Visitor<void> with VisitorVoidMixin {
+class ScopeOffsetValidator extends Visitor<void> {
   int classCount = 0;
   int memberCount = 0;
   int blockCount = 0;

pkg/dev_compiler/test/nullable_inference_test.dart

@@ -536,7 +536,7 @@ Future expectAllNotNull(String code) async {
 bool useAnnotations = false;
 NullableInference inference;
 
-class _TestRecursiveVisitor extends RecursiveVisitor {
+class _TestRecursiveVisitor extends RecursiveVisitor<void> {
   final Set<Library> librariesFromDill;
   int _functionNesting = 0;
   TypeEnvironment _typeEnvironment;

pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart

@@ -792,7 +792,7 @@ class ConstantsTransformer extends Transformer {
   }
 }
 
-class ConstantEvaluator extends RecursiveResultVisitor<Constant> {
+class ConstantEvaluator extends RecursiveVisitor<Constant> {
   final ConstantsBackend backend;
   final NumberSemantics numberSemantics;
   ConstantIntFolder intFolder;

pkg/front_end/lib/src/testing/id_extractor.dart

@@ -33,8 +33,7 @@ TreeNode computeTreeNodeWithOffset(TreeNode node) {
 
 /// Abstract visitor for computing data corresponding to a node or element,
 /// and record it with a generic [Id]
-abstract class DataExtractor<T> extends Visitor<void>
-    with VisitorVoidMixin, DataRegistry<T> {
+abstract class DataExtractor<T> extends Visitor with DataRegistry<T> {
   @override
   final Map<Id, ActualData<T>> actualMap;
 

pkg/front_end/test/comments_on_certain_arguments_tool.dart

@@ -159,7 +159,7 @@ api.CompilerOptions getOptions() {
   return options;
 }
 
-class InvocationVisitor extends RecursiveVisitor {
+class InvocationVisitor extends RecursiveVisitor<void> {
   void visitProcedure(Procedure node) {
     if (node.isNoSuchMethodForwarder) return;
     super.visitProcedure(node);

pkg/front_end/test/fasta/assert_locations_test.dart

@@ -102,7 +102,7 @@ void main() {}
 
 /// Visitor that verifies that all [AssertStatement]s in the Kernel AST
 /// have expected spans for their conditions.
-class VerifyingVisitor extends RecursiveVisitor {
+class VerifyingVisitor extends RecursiveVisitor<Null> {
   final Test test;
 
   /// Set of names of verified [Procedure]s.

pkg/front_end/test/fasta/testing/suite.dart

@@ -109,15 +109,14 @@ import 'package:kernel/ast.dart'
         TreeNode,
         UnevaluatedConstant,
         Version,
-        Visitor,
-        VisitorVoidMixin;
+        Visitor;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
 import 'package:kernel/core_types.dart' show CoreTypes;
 
 import 'package:kernel/kernel.dart'
-    show RecursiveResultVisitor, loadComponentFromBytes;
+    show RecursiveVisitor, loadComponentFromBytes;
 
 import 'package:kernel/reference_from_index.dart' show ReferenceFromIndex;
 
@@ -863,7 +862,7 @@ class StressConstantEvaluatorStep
   }
 }
 
-class StressConstantEvaluatorVisitor extends RecursiveResultVisitor<Node>
+class StressConstantEvaluatorVisitor extends RecursiveVisitor<Node>
     implements ErrorReporter {
   ConstantEvaluator constantEvaluator;
   ConstantEvaluator constantEvaluatorWithEmptyEnvironment;
@@ -1917,7 +1916,7 @@ class Verify extends Step<ComponentResult, ComponentResult, FastaContext> {
 /// Visitor that checks that the component has been transformed properly.
 // TODO(johnniwinther): Add checks for all nodes that are unsupported after
 // transformation.
-class VerifyTransformed extends Visitor<void> with VisitorVoidMixin {
+class VerifyTransformed extends Visitor<void> {
   final Target target;
   List<String> errors = [];
 

pkg/front_end/test/fasta/type_inference/type_schema_test.dart

@@ -113,7 +113,7 @@ class UnknownTypeTest {
   }
 }
 
-class _OrdinaryVisitor<R> extends Visitor<R> with VisitorNullMixin<R> {
+class _OrdinaryVisitor<R> extends Visitor<R> {
   final _UnaryFunction<DartType, R> _defaultDartType;
 
   _OrdinaryVisitor({_UnaryFunction<DartType, R> defaultDartType})
@@ -129,7 +129,7 @@ class _OrdinaryVisitor<R> extends Visitor<R> with VisitorNullMixin<R> {
   }
 }
 
-class _TypeSchemaVisitor<R> extends Visitor<R> with VisitorNullMixin<R> {
+class _TypeSchemaVisitor<R> extends Visitor<R> {
   final _UnaryFunction<DartType, R> _defaultDartType;
   final _UnaryFunction<UnknownType, R> _visitUnknownType;
 

pkg/front_end/test/static_types/analysis_helper.dart

@@ -42,7 +42,7 @@ run(Uri entryPoint, String allowedListPath,
       .run(verbose: verbose, generate: generate);
 }
 
-class StaticTypeVisitorBase extends RecursiveVisitor {
+class StaticTypeVisitorBase extends RecursiveVisitor<void> {
   final TypeEnvironment typeEnvironment;
 
   StaticTypeContext staticTypeContext;

pkg/frontend_server/lib/src/to_string_transformer.dart

@@ -11,7 +11,7 @@ import '../frontend_server.dart';
 
 /// A [RecursiveVisitor] that replaces [Object.toString] overrides with
 /// `super.toString()`.
-class ToStringVisitor extends RecursiveVisitor {
+class ToStringVisitor extends RecursiveVisitor<void> {
   /// The [packageUris] must not be null.
   ToStringVisitor(this._packageUris) : assert(_packageUris != null);
 

pkg/kernel/lib/binary/ast_to_binary.dart

@@ -2711,7 +2711,7 @@ class SwitchCaseIndexer {
   int operator [](SwitchCase node) => index[node];
 }
 
-class ConstantIndexer extends RecursiveResultVisitor {
+class ConstantIndexer extends RecursiveVisitor {
   final StringIndexer stringIndexer;
 
   final List<Constant> entries = <Constant>[];