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

pkg/compiler/lib/src/universe/class_set.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 dart2js.world.class_set;
 
 import 'dart:collection' show
     IterableBase;
-import '../common.dart';
 import '../elements/elements.dart' show
     ClassElement;
 import '../util/enumset.dart' show
     EnumSet;
 import '../util/util.dart' show
     Link;
 
 /// Enum for the different kinds of instantiation of a class.
 enum Instantiation {
   UNINSTANTIATED,
@@ skipping 842 matching lines @@
   /// Iteration stops immediately.
   STOP,
   /// Iteration skips the subclasses of the current class.
   SKIP_SUBCLASSES,
 }
 
 /// Visiting function used for the `forEachX` functions of [ClassHierarchyNode]
 /// and [ClassSet]. The return value controls the continued iteration. If `null`
 /// is returned, iteration continues to the end.
 typedef ForEach ForEachFunction(ClassElement cls);