Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/dart2jslib.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;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 import 'dart:profiler' show
     UserTag;
@@ skipping 16 matching lines @@
          LibraryElementX,
          PrefixElementX,
          VoidElementX,
          AnalyzableElement,
          DeferredLoaderGetterElementX;
 import 'helpers/helpers.dart';  // Included for debug helpers.
 import 'js/js.dart' as js;
 import 'js_backend/js_backend.dart' as js_backend;
 import 'library_loader.dart'
     show LibraryLoader,
-         LibraryLoaderTask;
+         LibraryLoaderTask,
+         LoadedLibraries;
 import 'mirrors_used.dart' show MirrorUsageAnalyzerTask;
 import 'native/native.dart' as native;
 import 'ordered_typeset.dart';
 import 'patch_parser.dart';
 import 'resolution/class_members.dart' show MembersCreator;
 import 'resolution/resolution.dart';
 import 'scanner/scannerlib.dart';
 import 'ssa/ssa.dart';
 import 'source_file.dart' show SourceFile;
 import 'tracer.dart' show Tracer;
 import 'tree/tree.dart';
 import 'types/types.dart' as ti;
 import 'universe/universe.dart';
 import 'util/characters.dart' show $_;
+import 'util/uri_extras.dart' as uri_extras show relativize;
 import 'util/util.dart';
 
 export 'helpers/helpers.dart';
 export 'resolution/resolution.dart' show TreeElements, TreeElementMapping;
 export 'scanner/scannerlib.dart' show isUserDefinableOperator,
                                       isUnaryOperator,
                                       isBinaryOperator,
                                       isTernaryOperator,
                                       isMinusOperator;
 export 'universe/universe.dart' show Selector, TypedSelector;
 export 'util/util.dart'
     show Spannable,
          CURRENT_ELEMENT_SPANNABLE,
          NO_LOCATION_SPANNABLE;
 
 part 'code_buffer.dart';
 part 'compile_time_constants.dart';
 part 'compiler.dart';
 part 'constant_system.dart';
 part 'constant_system_dart.dart';
 part 'diagnostic_listener.dart';
 part 'enqueue.dart';
 part 'resolved_visitor.dart';
 part 'script.dart';
 part 'typechecker.dart';
 part 'warnings.dart';
 part 'world.dart';