Add WorkItemBuilder to abstract WorkItem creation from the enqueuers.

pkg/compiler/lib/src/world.dart

 // Copyright (c) 2012, 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.world;
 
 import 'cache_strategy.dart';
 import 'closure.dart' show SynthesizedCallMethodElementX;
 import 'common/backend_api.dart' show BackendClasses;
 import 'common.dart';
 import 'core_types.dart' show CoreTypes, CoreClasses, CommonElements;
 import 'dart_types.dart';
 import 'elements/elements.dart'
     show
         ClassElement,
         Element,
         FunctionElement,
+        MemberElement,
         MixinApplicationElement,
         TypedefElement,
         FieldElement;
 import 'js_backend/backend.dart' show JavaScriptBackend;
 import 'ordered_typeset.dart';
 import 'types/masks.dart' show CommonMasks, FlatTypeMask, TypeMask;
 import 'universe/class_set.dart';
 import 'universe/function_set.dart' show FunctionSet;
 import 'universe/selector.dart' show Selector;
 import 'universe/side_effects.dart' show SideEffects;
@@ skipping 303 matching lines @@
   void registerClosureClass(ClassElement cls);
 }
 
 abstract class OpenWorld implements World {
   /// Called to add [cls] to the set of known classes.
   ///
   /// This ensures that class hierarchy queries can be performed on [cls] and
   /// classes that extend or implement it.
   void registerClass(ClassElement cls);
 
-  void registerUsedElement(Element element);
+  void registerUsedElement(MemberElement element);
   void registerTypedef(TypedefElement typedef);
 
   ClosedWorld closeWorld(DiagnosticReporter reporter);
 
   /// Returns an iterable over all mixin applications that mixin [cls].
   Iterable<MixinApplicationElement> allMixinUsesOf(ClassElement cls);
 }
 
 class WorldImpl implements ClosedWorld, ClosedWorldRefiner, OpenWorld {
   bool _closed = false;
@@ skipping 875 matching lines @@
   /// Only the class itself is included.
   EXACT,
 
   /// The class and all subclasses (transitively) are included.
   SUBCLASS,
 
   /// The class and all classes that implement or subclass it (transitively)
   /// are included.
   SUBTYPE,
 }