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

mojo/dart/packages/mojo_services/lib/http_server/http_request.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 http_request_mojom;
 import 'package:mojo/bindings.dart' as bindings;
 import 'package:mojo/core.dart' as core;
 
 
 
 
 class HttpRequest extends bindings.Struct {
   static const List<bindings.StructDataHeader> kVersions = const [
@@ skipping 152 matching lines @@
   }
 
   Map toJson() {
     throw new bindings.MojoCodecError(
         'Object containing handles cannot be encoded to JSON.');
   }
 }