dart2dart: Bugfix in copy propagator.

pkg/compiler/lib/src/js_backend/codegen/codegen.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.
 
 library code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
@@ skipping 383 matching lines @@
     js.Statement result = buildBody();
     if (usedLabels.remove(label)) {
       result = new js.LabeledStatement(label.name, result);
     }
     fallthrough = savedFallthrough;
     return result;
   }
 
   @override
   void visitAssign(tree_ir.Assign node) {
-    tree_ir.Expression value = node.definition;
+    tree_ir.Expression value = node.value;
     js.Expression definition = visitExpression(value);
 
     accumulator.add(new js.ExpressionStatement(new js.Assignment(
         buildVariableAccess(node.variable),
         definition)));
     visitStatement(node.next);
   }
 
   @override
   void visitBreak(tree_ir.Break node) {
@@ skipping 153 matching lines @@
 
   @override
   visitSuperInitializer(tree_ir.SuperInitializer node) {
     return errorUnsupportedNode(node);
   }
 
   errorUnsupportedNode(tree_ir.DartSpecificNode node) {
     throw "Unsupported node in JS backend: $node";
   }
 }