Use closure conversion in new dart2js backend.

pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart

 // Copyright (c) 2014, 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.
 
 part of tree_ir.optimization;
 
 /// Rewrites [WhileTrue] statements with an [If] body into a [WhileCondition],
 /// in situations where only one of the branches contains a [Continue] to the
 /// loop. Schematically:
 ///
@@ skipping 109 matching lines @@
 
   Statement visitFunctionDeclaration(FunctionDeclaration node) {
     new LoopRewriter().rewrite(node.definition);
     node.next = visitStatement(node.next);
     return node;
   }
 
   void visitFunctionExpression(FunctionExpression node) {
     new LoopRewriter().rewrite(node.definition);
   }
+
+  Statement visitSetField(SetField node) {
+    visitExpression(node.object);
+    visitExpression(node.value);
+    node.next = visitStatement(node.next);
+    return node;
+  }
+
 }