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

mojo/dart/packages/mojo/lib/mojo/http_header.mojom.dart

Index: mojo/dart/packages/mojo/lib/mojo/http_header.mojom.dart
diff --git a/mojo/dart/packages/mojo/lib/mojo/http_header.mojom.dart b/mojo/dart/packages/mojo/lib/mojo/http_header.mojom.dart
index 855399ee540abda57a952aed356b0927b9e46bb7..3fac12b833ff6dd7cfb50539e01caa3b00bc537d 100644
--- a/mojo/dart/packages/mojo/lib/mojo/http_header.mojom.dart
+++ b/mojo/dart/packages/mojo/lib/mojo/http_header.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 http_header_mojom;
 import 'package:mojo/bindings.dart' as bindings;