Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/patch_parser.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("patchparser");
 #import("dart:uri");
 
 #import("tree/tree.dart", prefix: "tree");
 #import("leg.dart", prefix: 'leg');  // CompilerTask, Compiler.
 #import("apiimpl.dart");
@@ skipping 60 matching lines @@
     assert(listener.nodes.isEmpty());
     return element.cachedNode;
   }
 }
 
 /**
  * Extension of the [Listener] interface to handle the extra "patch" pseudo-
  * keyword in patch files.
  * Patch files shouldn't have a type named "patch".
  */
-interface PatchListener extends Listener {
+abstract class PatchListener extends Listener {
   void beginPatch(Token patch);
   void endPatch(Token patch);
 }
 
 /**
  * Partial parser that extends the top-level and class grammars to allow the
  * word "patch" in front of some declarations.
  */
 class PatchParser extends PartialParser {
   PatchParser(PatchListener listener) : super(listener);
@@ skipping 198 matching lines @@
     super.addMember(element);
   }
 }
 
 // TODO(ahe): Get rid of this class.
 class PatchMetadataAnnotation extends MetadataAnnotation {
   final leg.Constant value = null;
 
   PatchMetadataAnnotation() : super(STATE_DONE);
 }