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

pkg/compiler/lib/src/helpers/expensive_set.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;
+import 'dart:collection';
 
 /**
  * The expensive set is a data structure useful for tracking down
  * excessive memory usage due to large sets. It acts as an ordinary
  * hash set, but it uses 10 times more memory (by default).
  */
 class ExpensiveSet<E> extends IterableBase<E> implements Set<E> {
 
   final List _sets;
 
@@ skipping 107 matching lines @@
   Set<E> toSet() {
     var result = new ExpensiveSet<E>(_sets.length);
     for (int i = 0; i < _sets.length; i++) {
       result._sets[i] = _sets[i].toSet();
     }
     return result;
   }
 
   String toString() => "expensive(${_sets[0]}x${_sets.length})";
 }