Make Enqueuer a pure interface.

tests/compiler/dart2js/exit_code_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 the exit code of dart2js in case of exceptions, errors, warnings, etc.
 
 
 import 'dart:async';
 import 'dart:io' show Platform;
 
@@ skipping 61 matching lines @@
   Future onLibrariesLoaded(LoadedLibraries loadedLibraries) {
     test('Compiler.onLibrariesLoaded');
     return super.onLibrariesLoaded(loadedLibraries);
   }
 
   WorldImpact analyzeElement(Element element) {
     test('Compiler.analyzeElement');
     return super.analyzeElement(element);
   }
 
-  WorldImpact codegen(CodegenWorkItem work, CodegenEnqueuer world) {
+  WorldImpact codegen(CodegenWorkItem work, Enqueuer world) {
     test('Compiler.codegen');
     return super.codegen(work, world);
   }
 
   test(String marker) {
     if (marker == testMarker) {
       switch (testType) {
       case 'assert':
         onTest(testMarker, testType);
         assert(false);
@@ skipping 194 matching lines @@
 
       expected = _expectedExitCode(
           beforeRun: tests[marker], fatalWarnings: true);
       totalExpectedErrors += expected.length;
       await testExitCodes(marker, expected, ['--fatal-warnings']);
     }
 
     Expect.equals(totalExpectedErrors, checkedResults);
   });
 }