Split loader from the rest of the compiler. This adds several abstractions to

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 '../common/resolution.dart' show
     ResolutionWorkItem;
 import '../common/tasks.dart' show
     CompilerTask;
 import '../common/work.dart' show
     ItemCompilationContext;
 import '../compiler.dart' show
     Compiler;
 import '../elements/elements.dart';
 import '../enqueue.dart' show
     ResolutionEnqueuer;
 import '../universe/world_impact.dart' show
     WorldImpact;
 
+/// A deserializer that can load a library element by reading it's information
+/// from a serialized form.
+abstract class LibraryDeserializer {
+  /// Loads the [LibraryElement] associated with a library under [uri], or null
+  /// if no serialized information is available for the given library.
+  LibraryElement readLibrary(Uri uri);
+}
+
 /// Task that supports deserialization of elements.
-class SerializationTask extends CompilerTask {
+class SerializationTask extends CompilerTask implements LibraryDeserializer {
   SerializationTask(Compiler compiler) : super(compiler);
 
   DeserializerSystem deserializer;
 
   String get name => 'Serialization';
 
   /// If `true`, data must be retained to support serialization.
   // TODO(johnniwinther): Make this more precise in terms of what needs to be
   // retained, for instance impacts, resolution data etc.
   bool supportSerialization = false;
@@ skipping 43 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);
 }