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

mojo/dart/packages/mojo_services/lib/activity/activity.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/activity/activity.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/activity/activity.mojom.dart b/mojo/dart/packages/mojo_services/lib/activity/activity.mojom.dart
index 43a4932b53a598e5d20ff523ba319d038955b381..0283785b89285319ce6d32649522a2e46e97fd8e 100644
--- a/mojo/dart/packages/mojo_services/lib/activity/activity.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/activity/activity.mojom.dart
@@ -1,6 +1,5 @@
-// 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 activity_mojom;
 import 'dart:async';