fix inconsistent firing of events for extension fields

test/event_test.dart

 // Copyright (c) 2015, 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.
 
 /// Tests event delivery using PbEventMixin.
 library event_test;
 
 import 'dart:typed_data' show Uint8List;
 
-import 'package:protobuf/protobuf.dart' show GeneratedMessage;
+import 'package:protobuf/protobuf.dart'
+    show GeneratedMessage, Extension, ExtensionRegistry, FieldType;
 import 'package:protobuf/src/protobuf/mixins/event_mixin.dart'
     show PbEventMixin, PbFieldChange;
 import 'package:test/test.dart' show test, expect, predicate, same;
 
 import 'mock_util.dart' show MockMessage;
 
 class Rec extends MockMessage with PbEventMixin {
   get className => "Rec";
   Rec create() => new Rec();
 }
 
+Extension comment = new Extension("Rec", "comment", 5, FieldType.OS);
+
 main() {
   test('Events are sent when setting and clearing a non-repeated field', () {
     var log = makeLog();
     var r = new Rec();
     r.changes.listen((List<PbFieldChange> changes) {
       log.add(changes);
     });
 
     r.val = 123;
     r.deliverChanges();
     checkLog(log, [[[1, 42, 123]]]);
 
     r.val = 456;
     r.deliverChanges();
     checkLog(log, [[[1, 123, 456]]]);
 
     r.val = 456; // no change
     r.deliverChanges();
     checkLog(log, []);
 
     r.clearField(1);
     r.deliverChanges();
     checkLog(log, [[[1, 456, 42]]]);
 
-    r.clearField(1);
+    r.clearField(1); // no change
     r.deliverChanges();
-    checkLog(log, []); // no change
+    checkLog(log, []);
   });
 
   test('Events are sent when creating and clearing a repeated field', () {
     var log = makeLog();
     var r = new Rec();
     r.changes.listen((List<PbFieldChange> changes) {
       log.add(changes);
     });
 
     // Accessing a repeated field replaces the default,
@@ skipping 88 matching lines @@
       log.add(changes);
     });
 
     r.mergeFromBuffer(bytes);
     // The changes should not include the repeated message.
     r.deliverChanges();
     checkLog(log, [
       [[1, 42, 123], [2, '', "hello"], [3, "<msg>", "<msg>"], [4, [], [456]]]
     ]);
   });
+
+  test('Events are not sent (yet) for extensions', () {
+    var log = makeLog();
+    var r = new Rec();
+    r.changes.listen((List<PbFieldChange> changes) {
+      log.add(changes);
+    });
+
+    r.setExtension(comment, "hello");
+    expect(r.getExtension(comment), "hello");
+    r.deliverChanges();
+    checkLog(log, []);
+
+    r.clearExtension(comment);
+    r.deliverChanges();
+    checkLog(log, []);
+
+    r.setExtension(comment, "hello");
+    expect(r.getExtension(comment), "hello");
+    r.clear();
+    r.deliverChanges();
+    checkLog(log, []);
+
+    r.setExtension(comment, "hello");
+    expect(r.getExtension(comment), "hello");
+    r.clearField(comment.tagNumber);
+    r.deliverChanges();
+    checkLog(log, []);
+
+    var registry = new ExtensionRegistry()..add(comment);
+    r.mergeFromJson('{"5": "hello"}', registry);
+    expect(r.getExtension(comment), "hello");
+    r.clear();
+    r.deliverChanges();
+    checkLog(log, []);
+
+    var src = new Rec()..setExtension(comment, "hello");
+    r.mergeFromMessage(src);
+    expect(r.getExtension(comment), "hello");
+    r.clear();
+    r.deliverChanges();
+    checkLog(log, []);
+
+    Uint8List bytes = src.writeToBuffer();
+    r.mergeFromBuffer(bytes, registry);
+    expect(r.getExtension(comment), "hello");
+    r.clear();
+    r.deliverChanges();
+    checkLog(log, []);
+  });
+
 }
 
 List<List<PbFieldChange>> makeLog() => <List<PbFieldChange>>[];
 
 void checkLog(List<List<PbFieldChange>> log, List<List<List>> expected) {
   var actual = <List<List>>[];
   for (var list in log) {
     actual.add(list.map(toTuple).toList());
   }
   log.clear();
   expect(actual, expected);
 }
 
 void checkHasAllFields(Rec r, bool expected) {
   expect(r.hasField(1), expected);
   expect(r.hasField(2), expected);
   expect(r.hasField(3), expected);
   expect(r.hasField(4), expected);
 }
 
 List toTuple(PbFieldChange fc) {
   fixValue(v) {
     if (v is GeneratedMessage) {
       return "<msg>";
     }
     return v;
   }
   return [fc.tag, fixValue(fc.oldValue), fixValue(fc.newValue)];
 }