Make Enqueuer a pure interface.

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, LibraryElement;
-import '../enqueue.dart'
-    show Enqueuer, EnqueueTask, CodegenEnqueuer, ResolutionEnqueuer;
+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;
 import '../serialization/serialization.dart'
     show DeserializerPlugin, SerializerPlugin;
@@ skipping 43 matching lines @@
   Iterable<ClassElement> classesRequiredForReflection = const [];
 
   // Given a [FunctionElement], return a buffer with the code generated for it
   // or null if no code was generated.
   CodeBuffer codeOf(Element element) => null;
 
   void initializeHelperClasses() {}
 
   void enqueueHelpers(ResolutionEnqueuer world, Registry registry);
 
-  /// Creates a [CodegenEnqueuer] specific to this backend.
-  CodegenEnqueuer createCodegenEnqueuer(Compiler compiler);
+  /// Creates an [Enqueuer] for code generation specific to this backend.
+  Enqueuer createCodegenEnqueuer(Compiler compiler);
 
   WorldImpact codegen(CodegenWorkItem work);
 
   // The backend determines the native resolution enqueuer, with a no-op
   // default, so tools like dart2dart can ignore the native classes.
   native.NativeEnqueuer nativeResolutionEnqueuer(world) {
     return new native.NativeEnqueuer();
   }
 
   native.NativeEnqueuer nativeCodegenEnqueuer(world) {
@@ skipping 312 matching lines @@
   }
 }
 
 /// Interface for serialization of backend specific data.
 class BackendSerialization {
   const BackendSerialization();
 
   SerializerPlugin get serializer => const SerializerPlugin();
   DeserializerPlugin get deserializer => const DeserializerPlugin();
 }