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

mojo/dart/packages/mojo_services/lib/tracing/tracing.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 tracing_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 _TraceProviderStartTracingParams extends bindings.Struct {
@@ skipping 1104 matching lines @@
   void start(core.MojoDataPipeProducer stream, String categories) {
     return impl.start(stream, categories);
   }
   void stopAndFlush() {
     return impl.stopAndFlush();
   }
 }