Fix hints reported by analyzer.

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 'dart:async' show EventSink, Future;
 
 import '../common/resolution.dart' show ResolutionImpact, ResolutionWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../elements/elements.dart';
-import '../enqueue.dart' show ResolutionEnqueuer;
 import '../universe/world_impact.dart' show WorldImpact;
 import 'json_serializer.dart';
 import 'serialization.dart';
 import 'system.dart';
 
 /// 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.
@@ skipping 127 matching lines @@
 /// elements.
 abstract class DeserializerSystem {
   Future<LibraryElement> readLibrary(Uri resolvedUri);
   bool isDeserialized(Element element);
   bool hasResolvedAst(ExecutableElement element);
   ResolvedAst getResolvedAst(ExecutableElement element);
   bool hasResolutionImpact(Element element);
   ResolutionImpact getResolutionImpact(Element element);
   WorldImpact computeWorldImpact(Element element);
 }