Make JavaScriptBackend.processAnnotations element-model agnostic

pkg/compiler/lib/src/common_elements.dart

 // Copyright (c) 2014, 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.
 
 // TODO(sigmund): rename and move to common/elements.dart
 library dart2js.type_system;
 
 import 'common/names.dart' show Identifiers, Uris;
-import 'js_backend/constant_system_javascript.dart';
+import 'constants/values.dart';
 import 'elements/types.dart';
 import 'elements/elements.dart' show PublicName;
 import 'elements/entities.dart';
 import 'js_backend/backend.dart' show JavaScriptBackend;
+import 'js_backend/constant_system_javascript.dart';
 import 'universe/call_structure.dart' show CallStructure;
 import 'universe/selector.dart' show Selector;
 import 'universe/call_structure.dart';
 
 /// The common elements and types in Dart.
 class CommonElements {
   final ElementEnvironment _env;
 
   CommonElements(this._env);
 
@@ skipping 1228 matching lines @@
   // TODO(johnniwinther): Remove this when the resolver is removed.
   DartType getUnaliasedType(DartType type);
 
   /// Returns the [CallStructure] corresponding to calling [entity] with all
   /// arguments, both required and optional.
   CallStructure getCallStructure(FunctionEntity entity);
 
   /// Returns `true` if [member] a the synthetic getter `loadLibrary` injected
   /// on deferred libraries.
   bool isDeferredLoadLibraryGetter(MemberEntity member);
+
+  /// Returns the metadata constants declared on [member].
+  Iterable<ConstantValue> getMemberMetadata(MemberEntity member);
 }