Added VariableUse expression to tree IR.

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 326 matching lines @@
     return new js.This();
   }
 
   @override
   js.Expression visitTypeOperator(tree_ir.TypeOperator node) {
     return giveup(node);
     // TODO: implement visitTypeOperator
   }
 
   @override
-  js.Expression visitVariable(tree_ir.Variable node) {
-    return new js.VariableUse(getVariableName(node));
+  js.Expression visitVariableUse(tree_ir.VariableUse node) {
+    return buildVariableAccess(node.variable);
+  }
+
+  js.Expression buildVariableAccess(tree_ir.Variable variable) {
+    return new js.VariableUse(getVariableName(variable));
   }
 
   @override
   void visitContinue(tree_ir.Continue node) {
     tree_ir.Statement fallthrough = this.fallthrough;
     if (node.target.binding == fallthrough) {
       // Fall through to continue target
     } else if (fallthrough is tree_ir.Continue &&
                fallthrough.target == node.target) {
       // Fall through to equivalent continue
@@ skipping 37 matching lines @@
     fallthrough = savedFallthrough;
     return result;
   }
 
   @override
   void visitAssign(tree_ir.Assign node) {
     tree_ir.Expression value = node.definition;
     js.Expression definition = visitExpression(value);
 
     accumulator.add(new js.ExpressionStatement(new js.Assignment(
-        visitVariable(node.variable),
+        buildVariableAccess(node.variable),
         definition)));
     visitStatement(node.next);
   }
 
   @override
   void visitBreak(tree_ir.Break node) {
     tree_ir.Statement fallthrough = this.fallthrough;
     if (node.target.binding.next == fallthrough) {
       // Fall through to break target
     } else if (fallthrough is tree_ir.Break &&
@@ skipping 111 matching lines @@
 
   @override
   visitSuperInitializer(tree_ir.SuperInitializer node) {
     return errorUnsupportedNode(node);
   }
 
   dynamic errorUnsupportedNode(tree_ir.DartSpecificNode node) {
     throw "Unsupported node in JS backend: $node";
   }
 }