Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/js_backend/js_backend.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 js_backend;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:collection' show Queue, LinkedHashMap, LinkedHashSet;
 
 import 'package:_internal/compiler/js_lib/shared/embedded_names.dart' as embeddedNames;
 
 import '../closure.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart2jslib.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
 import '../js_emitter/js_emitter.dart'
     show Emitter, CodeEmitterTask, ClassBuilder, MetadataEmitter;
-import '../library_loader.dart' show LibraryLoader;
+import '../library_loader.dart' show LibraryLoader, LoadedLibraries;
 import '../native/native.dart' as native;
 import '../ssa/ssa.dart';
 import '../tree/tree.dart';
 import '../types/types.dart';
 import '../universe/universe.dart';
 import '../util/characters.dart';
 import '../util/util.dart';
 
 import '../elements/visitor.dart' show
     ElementVisitor;
 
 part 'backend.dart';
 part 'checked_mode_helpers.dart';
 part 'constant_emitter.dart';
 part 'constant_handler_javascript.dart';
 part 'constant_system_javascript.dart';
 part 'custom_elements_analysis.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'native_emitter.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';