Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/diagnostic_listener.dart

Index: lib/compiler/implementation/diagnostic_listener.dart
diff --git a/lib/compiler/implementation/diagnostic_listener.dart b/lib/compiler/implementation/diagnostic_listener.dart
index 574f26a48d77cba3f2acda6b44b75308d4abe259..0fbc58b9209fcc30ae95958ca967f0f821a329d0 100644
--- a/lib/compiler/implementation/diagnostic_listener.dart
+++ b/lib/compiler/implementation/diagnostic_listener.dart
@@ -2,7 +2,7 @@
 // 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.
 
-interface DiagnosticListener {
+abstract class DiagnosticListener {
   // TODO(karlklose): replace cancel with better error reporting mechanism.
   void cancel([String reason, node, token, instruction, element]);
   // TODO(karlklose): rename log to something like reportInfo.