Use closure conversion in new dart2js backend.

pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.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;
 
 /// Eliminates moving assignments, such as w := v, by assigning directly to w
 /// at the definition of v.
 ///
 /// This compensates for suboptimal register allocation, and merges closure
@@ skipping 206 matching lines @@
     node.next = visitStatement(node.next);
     return node;
   }
 
   Statement visitExpressionStatement(ExpressionStatement node) {
     node.next = visitStatement(node.next);
     visitExpression(node.expression);
     return node;
   }
 
+  Statement visitSetField(SetField node) {
+    node.next = visitStatement(node.next);
+    visitExpression(node.value);
+    visitExpression(node.object);
+    return node;
+  }
+
   void visitFunctionExpression(FunctionExpression node) {
     new CopyPropagator().rewrite(node.definition);
   }
 
   void visitFieldInitializer(FieldInitializer node) {
     visitStatement(node.body);
   }
 
 }