Added VariableUse expression to tree IR.

pkg/compiler/lib/src/js_backend/codegen/js_tree_builder.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 js_tree_ir_builder;
 
 import '../../tree_ir/tree_ir_builder.dart' show Builder;
 import 'glue.dart' show Glue;
 import '../../dart2jslib.dart' show Selector, InternalErrorFunction;
 import '../../elements/elements.dart';
@@ skipping 18 matching lines @@
   }
 
   Selector get identicalSelector {
     return new Selector.call('identical', null, 2);
   }
 
   Expression visitIdentical(cps_ir.Identical node) {
     return new InvokeStatic(
         identicalFunction,
         identicalSelector,
-        <Expression>[getVariableReference(node.left),
-                     getVariableReference(node.right)]);
+        <Expression>[getVariableUse(node.left),
+                     getVariableUse(node.right)]);
   }
 
   Expression visitInterceptor(cps_ir.Interceptor node) {
     Element getInterceptor = _glue.getInterceptorMethod;
     String name = _glue.getInterceptorName(node.interceptedClasses);
     Selector selector = new Selector.call(name, null, 1);
     _glue.registerUseInterceptorInCodegen();
     return new InvokeStatic(
         getInterceptor,
         selector,
-        <Expression>[getVariableReference(node.input)]);
+        <Expression>[getVariableUse(node.input)]);
   }
 
   Expression visitGetField(cps_ir.GetField node) {
-    return new GetField(getVariableReference(node.object), node.field);
+    return new GetField(getVariableUse(node.object), node.field);
   }
 
   Statement visitSetField(cps_ir.SetField node) {
-    return new SetField(getVariableReference(node.object),
+    return new SetField(getVariableUse(node.object),
                         node.field,
-                        getVariableReference(node.value),
+                        getVariableUse(node.value),
                         visit(node.body));
   }
 
   Expression visitCreateBox(cps_ir.CreateBox node) {
     return new CreateBox();
   }
 
   Expression visitCreateInstance(cps_ir.CreateInstance node) {
     return new CreateInstance(
         node.classElement,
-        node.arguments.map(getVariableReference).toList());
+        node.arguments.map(getVariableUse).toList());
   }
 }