Use error-names for 'specializationOf'.

pkg/dart_messages/test/dart_messages_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.
 
 import 'dart:io' as io;
 
 import '../lib/shared_messages.dart';
 
 void testJsonIsUpdated() {
   var packageRoot = io.Platform.packageRoot;
@@ skipping 15 matching lines @@
 void testIdsAreUnique() {
   var usedIds = new Set();
   for (var entry in MESSAGES.values) {
     var id = "${entry.id}-${entry.subId}";
     if (!usedIds.add(id)) {
       throw "Id appears twice: $id";
     }
   }
 }
 
+void testSpecializationsAreOfSameId() {
+  for (var entry in MESSAGES.values) {
+    var specializationOf = entry.specializationOf;
+    if (specializationOf == null) continue;
+    var generic = MESSAGES[specializationOf];
+    if (generic == null) {
+      throw "More generic message doesn't exist: $specializationOf";
+    }
+    if (generic.id != entry.id) {
+      var id = "${entry.id}-${entry.subId}";
+      var genericId = "${generic.id}-${generic.subId}";
+      throw "Specialization doesn't have same id: $id - $genericId";
+    }
+  }
+}
+
 void main() {
   testJsonIsUpdated();
   testIdsAreUnique();
+  testSpecializationsAreOfSameId();
 }