pkg/compiler: fix imports

pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart

 // Copyright (c) 2015, 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.optimization.variable_merger;
 
+import '../tree_ir_nodes.dart';
 import 'optimization.dart' show Pass;
-import '../tree_ir_nodes.dart';
 
 /// Merges variables based on liveness and source variable information.
 ///
 /// This phase cleans up artifacts introduced by the translation through CPS,
 /// where each source variable is translated into several copies. The copies
 /// are merged again when they are not live simultaneously.
 class VariableMerger implements Pass {
   String get passName => 'Variable merger';
 
   final bool minifying;
@@ skipping 585 matching lines @@
     node.expression = visitExpression(node.expression);
     node.next = visitStatement(node.next);
     if (node.expression is VariableUse) {
       VariableUse use = node.expression;
       --use.variable.readCount;
       return node.next;
     }
     return node;
   }
 }