Improve the performance of closure-converted code.

pkg/kernel/lib/transformations/closure/rewriter.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library kernel.transformations.closure.rewriter;
 
 import '../../ast.dart';
 import 'converter.dart' show ClosureConverter;
 
 /// Used by the [Context] to initialize and update the context variable
 /// used to capture the variables closed over by functions.
 abstract class AstRewriter {
   /// The declared variable that holds the context.
   VariableDeclaration contextDeclaration;
 
   /// Expression that is used to initialize the vector representing the context.
   /// It's [length] field is modified by the [extend] operation
   VectorCreation vectorCreation;
 
   /// Creates a new [AstRewriter] for a (nested) [Block].
   BlockRewriter forNestedBlock(Block block);
 
   /// Inserts an allocation of a context and initializes [contextDeclaration]
   /// and [vectorCreation].
   void insertContextDeclaration(Expression accessParent);
 
   /// Inserts an expression or statement that extends the context.
   void insertExtendContext(VectorSet extender);
 
+  /// Inserts an expression that sets a parameter to NULL, so we don't have
+  /// unnecessary references to it.
+  void insertZeroOutParameter(VariableDeclaration parameter);
+
   void _createDeclaration() {
     assert(contextDeclaration == null && vectorCreation == null);
 
     // Context size is set to 2 initially, because the 0-th element of it holds
     // the vector of type arguments that the VM creates, and the 1-st element
     // works as a link to the parent context.
     vectorCreation = new VectorCreation(2);
     contextDeclaration = new VariableDeclaration.forValue(vectorCreation,
         type: const DynamicType());
     contextDeclaration.name = "#context";
@@ skipping 37 matching lines @@
     if (accessParent is! NullLiteral) {
       // Index 1 of a context always points to the parent.
       _insertStatement(new ExpressionStatement(
           new VectorSet(new VariableGet(contextDeclaration), 1, accessParent)));
     }
   }
 
   void insertExtendContext(VectorSet extender) {
     _insertStatement(new ExpressionStatement(extender));
   }
+
+  void insertZeroOutParameter(VariableDeclaration parameter) {
+    _insertStatement(
+        new ExpressionStatement(new VariableSet(parameter, new NullLiteral())));
+  }
 }
 
 class InitializerListRewriter extends AstRewriter {
   final Constructor parentConstructor;
   final List<Initializer> prefix = [];
 
   InitializerListRewriter(this.parentConstructor);
 
   @override
   BlockRewriter forNestedBlock(Block block) {
     return new BlockRewriter(block);
   }
 
   @override
   void insertContextDeclaration(Expression accessParent) {
     _createDeclaration();
     var init = new LocalInitializer(contextDeclaration);
     init.parent = parentConstructor;
     prefix.add(init);
   }
 
   @override
   void insertExtendContext(VectorSet extender) {
     var init = new LocalInitializer(
         new VariableDeclaration(null, initializer: extender));
     init.parent = parentConstructor;
     prefix.add(init);
   }
+
+  @override
+  void insertZeroOutParameter(VariableDeclaration parameter) {
+    var init = new LocalInitializer(new VariableDeclaration(null,
+        initializer: new VariableSet(parameter, new NullLiteral())));
+    init.parent = parentConstructor;
+    prefix.add(init);
+  }
 }