Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/dart2jslib.dart

Index: pkg/compiler/lib/src/dart2jslib.dart
diff --git a/pkg/compiler/lib/src/dart2jslib.dart b/pkg/compiler/lib/src/dart2jslib.dart
index a59cefccef7b7ae29acea80f79e6d2d90654cc18..01cd054abaf1e724674ce7a8e364c17eac5017e8 100644
--- a/pkg/compiler/lib/src/dart2jslib.dart
+++ b/pkg/compiler/lib/src/dart2jslib.dart
@@ -34,7 +34,8 @@ 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';
@@ -49,6 +50,7 @@ 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';