Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/js_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 js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1745 matching lines @@
         preserveUrisMarker = find(library, 'preserveUris');
         preserveLibraryNamesMarker = find(library, 'preserveLibraryNames');
       } else if (uri == DART_JS_NAMES) {
         preserveNamesMarker = find(library, 'preserveNames');
       } else if (uri == DART_HTML) {
         htmlLibraryIsLoaded = true;
       }
     });
   }
 
-  Future onLibrariesLoaded(Map<Uri, LibraryElement> loadedLibraries) {
-    if (!loadedLibraries.containsKey(Compiler.DART_CORE)) {
+  Future onLibrariesLoaded(LoadedLibraries loadedLibraries) {
+    if (!loadedLibraries.containsLibrary(Compiler.DART_CORE)) {
       return new Future.value();
     }
 
-    assert(loadedLibraries.containsKey(Compiler.DART_CORE));
-    assert(loadedLibraries.containsKey(DART_INTERCEPTORS));
-    assert(loadedLibraries.containsKey(DART_JS_HELPER));
+    assert(loadedLibraries.containsLibrary(Compiler.DART_CORE));
+    assert(loadedLibraries.containsLibrary(DART_INTERCEPTORS));
+    assert(loadedLibraries.containsLibrary(DART_JS_HELPER));
 
     if (jsInvocationMirrorClass != null) {
       jsInvocationMirrorClass.ensureResolved(compiler);
       invokeOnMethod = jsInvocationMirrorClass.lookupLocalMember(INVOKE_ON);
     }
 
     // [LinkedHashMap] is reexported from dart:collection and can therefore not
     // be loaded from dart:core in [onLibraryScanned].
     mapLiteralClass = compiler.coreLibrary.find('LinkedHashMap');
     assert(invariant(compiler.coreLibrary, mapLiteralClass != null,
@@ skipping 677 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }