Added VariableUse expression to tree IR.

pkg/compiler/lib/src/tree_ir/tree_ir_tracer.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 tree_ir_tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'tree_ir_nodes.dart';
 import 'optimization/optimization.dart';
@@ skipping 295 matching lines @@
   String expr(Expression e) {
     return e.accept(new SubexpressionVisitor(names));
   }
 }
 
 class SubexpressionVisitor extends ExpressionVisitor<String> {
   Names names;
 
   SubexpressionVisitor(this.names);
 
-  String visitVariable(Variable node) {
-    return names.varName(node);
+  String visitVariableUse(VariableUse node) {
+    return names.varName(node.variable);
   }
 
   String formatArguments(Invoke node) {
     List<String> args = new List<String>();
     int positionalArgumentCount = node.selector.positionalArgumentCount;
     for (int i = 0; i < positionalArgumentCount; ++i) {
       args.add(node.arguments[i].accept(this));
     }
     for (int i = 0; i < node.selector.namedArgumentCount; ++i) {
       String name = node.selector.namedArguments[i];
@@ skipping 156 matching lines @@
       String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
         name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }