dart2js: add initial support for lookup-maps

pkg/compiler/lib/src/common/backend_api.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.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../compiler.dart' show
     Compiler;
@@ skipping 321 matching lines @@
   /// resolution queue has drained and [recentClasses] will be a superset of the
   /// current value.
   ///
   /// There is no guarantee that a class is only present once in
   /// [recentClasses], but every class seen by the [enqueuer] will be present in
   /// [recentClasses] at least once.
   bool onQueueEmpty(Enqueuer enqueuer, Iterable<ClassElement> recentClasses) {
     return true;
   }
 
+  /// Called after the queue is closed. [onQueueEmpty] may be called multiple
+  /// times, but [onQueueClosed] is only called once.
+  void onQueueClosed() {}
+
   /// Called after [element] has been resolved.
   // TODO(johnniwinther): Change [TreeElements] to [Registry] or a dependency
   // node. [elements] is currently unused by the implementation.
   void onElementResolved(Element element, TreeElements elements) {}
 
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;
 
   FunctionElement helperForBadMain() => null;
 
   FunctionElement helperForMissingMain() => null;
 
   FunctionElement helperForMainArity() => null;
 
   void forgetElement(Element element) {}
 
   void registerMainHasArguments(Enqueuer enqueuer) {}
 
   void registerAsyncMarker(FunctionElement element,
                            Enqueuer enqueuer,
                            Registry registry) {}
 }