Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/helpers/expensive_map.dart

 // Copyright (c) 2013, 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 dart2js.helpers;
 
 /**
  * The expensive map is a data structure useful for tracking down
  * excessive memory usage due to large maps. It acts as an ordinary
  * hash map, but it uses 10 times more memory (by default).
  */
 class ExpensiveMap<K, V> implements Map<K, V> {
 
   final List _maps;
 
@@ skipping 48 matching lines @@
   }
 
   void clear() {
     for (int i = 0; i < _maps.length; i++) {
       _maps[i].clear();
     }
   }
 
   String toString() => "expensive(${_maps[0]}x${_maps.length})";
 }