Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/class_set_test.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.
 
 // Test for iterators on for [SubclassNode].
 
 library world_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'type_test_helper.dart';
 import 'package:compiler/src/elements/elements.dart'
        show Element, ClassElement;
-import 'package:compiler/src/dart2jslib.dart';
 import 'package:compiler/src/universe/class_set.dart';
 import 'package:compiler/src/util/util.dart';
+import 'package:compiler/src/world.dart';
 
 void main() {
   asyncTest(() => TypeEnvironment.create(r"""
       ///        A
       ///       / \
       ///      B   C
       ///     /   /|\
       ///    D   E F G
       ///
       class A {}
@@ skipping 288 matching lines @@
     checkState(A, currentNode: G, stack: [B]);
     Expect.equals(G, iterator.current);
     Expect.isTrue(iterator.moveNext());
     checkState(A, currentNode: D, stack: []);
     Expect.equals(D, iterator.current);
     Expect.isFalse(iterator.moveNext());
     checkState(A, currentNode: null, stack: []);
     Expect.isNull(iterator.current);
   }));
 }