Share the first error messages.

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';
+import 'dart:io' as io;
 
 import '../lib/shared_messages.dart';
 
 void testJsonIsUpdated() {
-  var packageRoot = Platform.packageRoot;
+  var packageRoot = io.Platform.packageRoot;
   if (packageRoot == null || packageRoot == "") {
     throw new UnsupportedError("This test requires a package root.");
   }
   var jsonUri = Uri.parse(packageRoot).resolve(
       'dart_messages/generated/shared_messages.json');
   var jsonPath = jsonUri.toFilePath();
-  var content = new File(jsonPath).readAsStringSync();
+  var content = new io.File(jsonPath).readAsStringSync();
   if (messagesAsJson != content) {
     print("The content of the Dart messages and the corresponding JSON file");
     print("is not the same.");
     print("Please run bin/publish.dart to update the JSON file.");
     throw "Content is not the same";
   }
 }
 
 void testIdsAreUnique() {
   var usedIds = new Set();
   for (var entry in MESSAGES.values) {
-    var id = entry.id;
+    var id = "${entry.id}-${entry.subId}";
     if (!usedIds.add(id)) {
       throw "Id appears twice: $id";
     }
   }
 }
 
 void main() {
   testJsonIsUpdated();
   testIdsAreUnique();
 }