Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/dart_backend/backend.dart

Index: pkg/compiler/lib/src/dart_backend/backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend.dart b/pkg/compiler/lib/src/dart_backend/backend.dart
index 3479b499f0224f31486c9527cd5919e8cc3068d5..174c58b99a97eb0f904308c538b12a60111466d9 100644
--- a/pkg/compiler/lib/src/dart_backend/backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend.dart
@@ -261,10 +261,10 @@ class DartBackend extends Backend {
 
   log(String message) => compiler.log('[DartBackend] $message');
 
-  Future onLibrariesLoaded(Map<Uri, LibraryElement> loadedLibraries) {
+  Future onLibrariesLoaded(LoadedLibraries loadedLibraries) {
     // All platform classes must be resolved to ensure that their member names
     // are preserved.
-    loadedLibraries.values.forEach((LibraryElement library) {
+    loadedLibraries.forEachLibrary((LibraryElement library) {
       if (library.isPlatformLibrary) {
         library.forEachLocalMember((Element element) {
           if (element.isClass) {
@@ -275,10 +275,10 @@ class DartBackend extends Backend {
       }
     });
     if (useMirrorHelperLibrary &&
-        loadedLibraries.containsKey(Compiler.DART_MIRRORS)) {
+        loadedLibraries.containsLibrary(Compiler.DART_MIRRORS)) {
       return compiler.libraryLoader.loadLibrary(
           compiler.translateResolvedUri(
-              loadedLibraries[Compiler.DART_MIRRORS],
+              loadedLibraries.getLibrary(Compiler.DART_MIRRORS),
               MirrorRenamerImpl.DART_MIRROR_HELPER, null)).
           then((LibraryElement library) {
         mirrorRenamer = new MirrorRenamerImpl(compiler, this, library);