Add WorkItemBuilder to abstract WorkItem creation from the enqueuers.

pkg/compiler/lib/src/common/backend_api.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.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
 import '../common/resolution.dart' show ResolutionImpact, Frontend, Target;
 import '../compile_time_constants.dart'
     show BackendConstantEnvironment, ConstantCompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart' show ConstantSystem;
 import '../constants/expressions.dart' show ConstantExpression;
 import '../constants/values.dart' show ConstantValue;
 import '../dart_types.dart' show DartType, InterfaceType;
 import '../elements/elements.dart'
-    show ClassElement, Element, FunctionElement, MethodElement, LibraryElement;
+    show
+        ClassElement,
+        Element,
+        FunctionElement,
+        MemberElement,
+        MethodElement,
+        LibraryElement;
 import '../elements/entities.dart';
 import '../enqueue.dart' show Enqueuer, EnqueueTask, ResolutionEnqueuer;
 import '../io/code_output.dart' show CodeBuffer;
 import '../io/source_information.dart' show SourceInformationStrategy;
 import '../js_backend/backend_helpers.dart' as js_backend show BackendHelpers;
 import '../js_backend/js_backend.dart' as js_backend;
 import '../library_loader.dart' show LibraryLoader, LoadedLibraries;
 import '../native/native.dart' as native show NativeEnqueuer, maybeEnableNative;
 import '../patch_parser.dart'
     show checkNativeAnnotation, checkJsInteropAnnotation;
@@ skipping 167 matching lines @@
     Uri canonicalUri = library.canonicalUri;
     if (canonicalUri == js_backend.BackendHelpers.DART_JS_HELPER ||
         canonicalUri == js_backend.BackendHelpers.DART_INTERCEPTORS) {
       return true;
     }
     return false;
   }
 
   /// Called to register that [element] is statically known to be used. Any
   /// backend specific [WorldImpact] of this is returned.
-  WorldImpact registerUsedElement(Element element, {bool forResolution}) =>
+  WorldImpact registerUsedElement(MemberElement element,
+          {bool forResolution}) =>
       const WorldImpact();
 
   /// This method is called immediately after the [LibraryElement] [library] has
   /// been created.
   void onLibraryCreated(LibraryElement library) {}
 
   /// This method is called immediately after the [library] and its parts have
   /// been scanned.
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader) {
     // TODO(johnniwinther): Move this to [JavaScriptBackend].
@@ skipping 187 matching lines @@
   ClassElement get asyncFutureImplementation;
   ClassElement get asyncStarStreamImplementation;
   ClassElement get indexableImplementation;
   ClassElement get mutableIndexableImplementation;
   ClassElement get indexingBehaviorImplementation;
 
   bool isDefaultEqualityImplementation(Element element);
   bool isInterceptorClass(ClassElement cls);
   bool isNative(Element element);
 }