Make Enqueuer a pure interface.

pkg/compiler/lib/src/common/codegen.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.common.codegen;
 
 import '../common.dart';
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart' show ConstantValue;
 import '../dart_types.dart' show DartType, InterfaceType;
 import '../elements/elements.dart'
     show
         AstElement,
         ClassElement,
         Element,
         FunctionElement,
         LocalFunctionElement,
         ResolvedAst;
-import '../enqueue.dart' show CodegenEnqueuer;
+import '../enqueue.dart' show Enqueuer;
 import '../universe/use.dart' show DynamicUse, StaticUse, TypeUse;
 import '../universe/world_impact.dart'
     show WorldImpact, WorldImpactBuilder, WorldImpactVisitor;
 import '../util/util.dart' show Pair, Setlet;
 import 'registry.dart' show Registry, EagerRegistry;
 import 'work.dart' show ItemCompilationContext, WorkItem;
 
 class CodegenImpact extends WorldImpact {
   const CodegenImpact();
 
@@ skipping 207 matching lines @@
         message: "$element has no resolved ast."));
     ResolvedAst resolvedAst = element.resolvedAst;
     return new CodegenWorkItem.internal(resolvedAst, compilationContext);
   }
 
   CodegenWorkItem.internal(
       ResolvedAst resolvedAst, ItemCompilationContext compilationContext)
       : this.resolvedAst = resolvedAst,
         super(resolvedAst.element, compilationContext);
 
-  WorldImpact run(Compiler compiler, CodegenEnqueuer world) {
+  WorldImpact run(Compiler compiler, Enqueuer world) {
     if (world.isProcessed(element)) return const WorldImpact();
 
     registry = new CodegenRegistry(compiler, element);
     return compiler.codegen(this, world);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }