pkg/compiler: fix imports

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';
 
 class Block {
   Label label;
   int index;
 
   /// Mixed list of [Statement] and [Block].
   /// A [Block] represents a synthetic goto statement.
   final List statements = [];
@@ skipping 593 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;
   }
 }