Dart: Add warning to the top of .mojom.dart files

mojo/dart/packages/mojo_services/lib/authentication/authentication_impl_db.mojom.dart

-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
+// WARNING: DO NOT EDIT. This file was generated by a program.
+// See $MOJO_SDK/tools/bindings/mojom_bindings_generator.py.
 
 library authentication_impl_db_mojom;
 import 'package:mojo/bindings.dart' as bindings;
 
 
 
 
 class Db extends bindings.Struct {
   static const List<bindings.StructDataHeader> kVersions = const [
     const bindings.StructDataHeader(24, 0)
@@ skipping 125 matching lines @@
   Map toJson() {
     Map map = new Map();
     map["version"] = version;
     map["lastSelectedAccounts"] = lastSelectedAccounts;
     return map;
   }
 }