Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/serialization/task.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.serialization.task;
 
 import '../dart2jslib.dart';
 import '../elements/elements.dart';
+import '../enqueue.dart' show
+    ResolutionEnqueuer,
+    WorldImpact;
 
 /// Task that supports deserialization of elements.
 class SerializationTask extends CompilerTask {
   SerializationTask(Compiler compiler) : super(compiler);
 
   DeserializerSystem deserializer;
 
   String get name => 'Serialization';
 
   /// Returns the [LibraryElement] for [resolvedUri] if available from
@@ skipping 41 matching lines @@
   }
 }
 
 /// The interface for a system that supports deserialization of libraries and
 /// elements.
 abstract class DeserializerSystem {
   LibraryElement readLibrary(Uri resolvedUri);
   bool isDeserialized(Element element);
   WorldImpact computeWorldImpact(Element element);
 }