Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/js_backend/codegen_listener.dart

 // Copyright (c) 2017, 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 js_backend.backend.codegen_listener;
 
-import '../common.dart';
 import '../common/backend_api.dart';
 import '../common/names.dart' show Identifiers;
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
 import '../constants/values.dart';
-import '../dump_info.dart' show DumpInfoTask;
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/resolution_types.dart';
 import '../enqueue.dart' show Enqueuer, EnqueuerListener;
 import '../native/enqueue.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/use.dart' show StaticUse, TypeUse;
 import '../universe/world_impact.dart'
     show WorldImpact, WorldImpactBuilder, WorldImpactBuilderImpl;
 import 'backend.dart';
 import 'backend_helpers.dart';
 import 'backend_impact.dart';
 import 'backend_usage.dart';
-import 'constant_handler_javascript.dart';
 import 'custom_elements_analysis.dart';
 import 'lookup_map_analysis.dart' show LookupMapAnalysis;
 import 'mirrors_analysis.dart';
-import 'mirrors_data.dart';
 import 'type_variable_handler.dart';
 
 class CodegenEnqueuerListener extends EnqueuerListener {
   final ElementEnvironment _elementEnvironment;
   final CommonElements _commonElements;
   final BackendHelpers _helpers;
   final BackendImpacts _impacts;
   final BackendClasses _backendClasses;
 
   final BackendUsage _backendUsage;
@@ skipping 300 matching lines @@
   WorldImpact registerInstantiatedClass(ClassEntity cls) {
     return _processClass(cls);
   }
 
   @override
   void logSummary(void log(String message)) {
     _lookupMapAnalysis.logSummary(log);
     _nativeEnqueuer.logSummary(log);
   }
 }