Emit warning on import of dart:mirrors.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 0f01f9c86572926652e2c1bf78a4637cc4c23fd5..893445bae1aa17f71bd731ed314e35f97f35f2ee 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1763,14 +1763,14 @@ class JavaScriptBackend extends Backend {
     });
   }
 
-  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);