Improve the performance of closure-converted code.

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

Index: pkg/kernel/lib/transformations/closure/rewriter.dart
diff --git a/pkg/kernel/lib/transformations/closure/rewriter.dart b/pkg/kernel/lib/transformations/closure/rewriter.dart
index dd139de35033c7d783f511db162d62772b1f0a00..d3ae56ee09e91b8c4b8617e2ccc1471055aeff9b 100644
--- a/pkg/kernel/lib/transformations/closure/rewriter.dart
+++ b/pkg/kernel/lib/transformations/closure/rewriter.dart
@@ -27,6 +27,10 @@ abstract class AstRewriter {
   /// 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);
 
@@ -84,6 +88,11 @@ class BlockRewriter extends AstRewriter {
   void insertExtendContext(VectorSet extender) {
     _insertStatement(new ExpressionStatement(extender));
   }
+
+  void insertZeroOutParameter(VariableDeclaration parameter) {
+    _insertStatement(
+        new ExpressionStatement(new VariableSet(parameter, new NullLiteral())));
+  }
 }
 
 class InitializerListRewriter extends AstRewriter {
@@ -112,4 +121,12 @@ class InitializerListRewriter extends AstRewriter {
     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);
+  }
 }