Added VariableUse expression to tree IR.

pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.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;
 
 /// Rewrites logical expressions to be more compact in the Tree IR.
 ///
 /// In this class an expression is said to occur in "boolean context" if
 /// its result is immediately applied to boolean conversion.
@@ skipping 159 matching lines @@
     node.next = visitStatement(node.next);
     return node;
   }
 
   Statement visitExpressionStatement(ExpressionStatement node) {
     node.expression = visitExpression(node.expression);
     node.next = visitStatement(node.next);
     return node;
   }
 
-
-  Expression visitVariable(Variable node) {
+  Expression visitVariableUse(VariableUse node) {
     return node;
   }
 
   Expression visitInvokeStatic(InvokeStatic node) {
     _rewriteList(node.arguments);
     return node;
   }
 
   Expression visitInvokeMethod(InvokeMethod node) {
     node.receiver = visitExpression(node.receiver);
@@ skipping 291 matching lines @@
   }
 
   /// Destructively updates each entry of [l] with the result of visiting it.
   void _rewriteList(List<Expression> l) {
     for (int i = 0; i < l.length; i++) {
       l[i] = visitExpression(l[i]);
     }
   }
 }