Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/dart_backend/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.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 243 matching lines @@
         nonPlatformSize += compilationUnit.script.file.length;
       }
     }
     int percentage = totalOutputSize * 100 ~/ nonPlatformSize;
     log('Total used non-platform files size: ${nonPlatformSize} bytes, '
         'Output total size: $totalOutputSize bytes (${percentage}%)');
   }
 
   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) {
             ClassElement classElement = element;
             classElement.ensureResolved(compiler);
           }
         });
       }
     });
     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);
       });
     }
     return new Future.value();
   }
 
   void registerStaticUse(Element element, Enqueuer enqueuer) {
     if (element == compiler.mirrorSystemGetNameFunction) {
@@ skipping 149 matching lines @@
   }
 
   ConstantExpression compileMetadata(MetadataAnnotation metadata,
                            Node node,
                            TreeElements elements) {
     return measure(() {
       return constantCompiler.compileMetadata(metadata, node, elements);
     });
   }
 }