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

mojo/dart/packages/mojo_services/lib/mojo/media/media_metadata.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 media_metadata_mojom;
 import 'package:mojo/bindings.dart' as bindings;
 
 
 
 
 class MediaMetadata extends bindings.Struct {
   static const List<bindings.StructDataHeader> kVersions = const [
     const bindings.StructDataHeader(64, 0)
@@ skipping 144 matching lines @@
     map["album"] = album;
     map["publisher"] = publisher;
     map["genre"] = genre;
     map["composer"] = composer;
     return map;
   }
 }