Move some core classes to collection library.

sdk/lib/_internal/compiler/implementation/types/types.dart

 // Copyright (c) 2012, 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 types;
 
+import 'dart:collection' show Queue;
+
 import '../dart2jslib.dart' hide Selector;
 import '../tree/tree.dart';
 import '../elements/elements.dart';
 import '../util/util.dart';
 import '../universe/universe.dart';
 
 part 'concrete_types_inferrer.dart';
 
 /**
  * The types task infers guaranteed types globally.
@@ skipping 225 matching lines @@
    * [:null:] if it gives up.
    */
   Element computeLub(Element a, Element b) {
     // Fast common case, but also simple initial implementation.
     if (identical(a, b)) return a;
 
     // TODO(ahe): Improve the following "computation"...
     return null;
   }
 }