Revert "Move dart2jslib parts into separate libraries."

tests/compiler/dart2js/world_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.
 
 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/world.dart' show ClassWorld;
+import 'package:compiler/src/dart2jslib.dart';
 
 void main() {
   asyncTest(() => TypeEnvironment.create(r"""
       class A {}
       class B {}
       class C_Super extends A {}
       class C extends C_Super {}
       class D implements A {}
       class E extends B implements A {}
       class F extends Object with A implements B {}
@@ skipping 120 matching lines @@
     testMixinUses(A, [F.superclass, G.superclass.superclass]);
     testMixinUses(B, [G.superclass]);
     testMixinUses(C, []);
     testMixinUses(D, []);
     testMixinUses(E, []);
     testMixinUses(F, []);
     testMixinUses(G, []);
 
   }));
 }