dart2js: add initial support for lookup-maps

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index 63b112e22c1c262a8afa981c1d3750eed2f80a6a..e715f29ab36996a46df81950816a11811e906a32 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -339,6 +339,10 @@ abstract class Backend {
     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.