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

mojo/dart/packages/mojo_services/lib/input/input.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 input_mojom;
 import 'dart:async';
 import 'package:mojo/bindings.dart' as bindings;
 import 'package:mojo/core.dart' as core;
 import 'package:mojo/mojo/bindings/types/service_describer.mojom.dart' as service_describer;
 
 
 
 class _InputClientOnBackButtonParams extends bindings.Struct {
@@ skipping 689 matching lines @@
   }
 
 
   void setClient(InputClientInterface client) {
     return impl.setClient(client);
   }
 }