Make Enqueuer a pure interface.

tests/compiler/dart2js/mirrors_used_test.dart

 // Copyright (c) 2013, 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.
 
 /// Test that the @MirrorsUsed annotation suppress hints and that only
 /// requested elements are retained for reflection.
 library dart2js.test.mirrors_used_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 40 matching lines @@
 
 void main() {
   asyncTest(() async {
     var result = await runCompiler(
       memorySourceFiles: MEMORY_SOURCE_FILES,
       diagnosticHandler: new LegacyCompilerDiagnostics(expectOnlyVerboseInfo),
       options: ['--enable-experimental-mirrors']);
     CompilerImpl compiler = result.compiler;
     print('');
     List generatedCode =
-        Elements.sortedByPosition(compiler.enqueuer.codegen.generatedCode.keys);
+        Elements.sortedByPosition(compiler.enqueuer.codegen.processedEntities);
     for (var element in generatedCode) {
       print(element);
     }
     print('');
 
     // This assertion can fail for two reasons:
     // 1. Too many elements retained for reflection.
     // 2. Some code was refactored, and there are more methods.
     // Either situation could be problematic, but in situation 2, it is often
     // acceptable to increase [expectedMethodCount] a little.
@@ skipping 113 matching lines @@
 library lib;
 
 import 'dart:mirrors';
 
 useReflect(type) {
   print(new Symbol('Foo'));
   print(MirrorSystem.getName(reflectClass(type).owner.qualifiedName));
 }
 """,
 };