Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/patch_parser.dart

Index: lib/compiler/implementation/patch_parser.dart
diff --git a/lib/compiler/implementation/patch_parser.dart b/lib/compiler/implementation/patch_parser.dart
index 2a719238d3ad19acec17868a0a2f02d794f8fd3f..cfc78f8531f9f1925856482c32bf1e7e5905a4b1 100644
--- a/lib/compiler/implementation/patch_parser.dart
+++ b/lib/compiler/implementation/patch_parser.dart
@@ -78,7 +78,7 @@ class PatchParserTask extends leg.CompilerTask {
  * 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);
 }