Added VariableUse expression to tree IR.

pkg/compiler/lib/src/tree_ir/optimization/loop_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 [WhileTrue] statements with an [If] body into a [WhileCondition],
 /// in situations where only one of the branches contains a [Continue] to the
 /// loop. Schematically:
 ///
@@ skipping 25 matching lines @@
   }
 
   Statement visitLabeledStatement(LabeledStatement node) {
     node.body = visitStatement(node.body);
     node.next = visitStatement(node.next);
     return node;
   }
 
   Statement visitAssign(Assign node) {
     // Clean up redundant assignments left behind in the previous phase.
-    if (node.variable == node.definition) {
+    Expression def = node.definition;
+    if (def is VariableUse && node.variable == def.variable) {
       --node.variable.readCount;
       --node.variable.writeCount;
       return visitStatement(node.next);
     }
     visitExpression(node.definition);
     node.next = visitStatement(node.next);
     return node;
   }
 
   Statement visitReturn(Return node) {
@@ skipping 72 matching lines @@
   }
 
   Statement visitSetField(SetField node) {
     visitExpression(node.object);
     visitExpression(node.value);
     node.next = visitStatement(node.next);
     return node;
   }
 
 }