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

mojo/public/tools/bindings/generators/dart_templates/module.lib.tmpl

-// 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.
+{#- 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 {{module.name|dot_to_underscore}};
 
 {%- if module.interfaces %}
 import 'dart:async';
 {%- endif %}
 {%- if should_gen_mojom_types %}
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:typed_data';
 {%- endif %}
 
 {%- if module.structs or module.unions or module.interfaces or module.enums or should_gen_mojom_types%}
 import 'package:mojo/bindings.dart' as bindings;
 {%- endif %}
 {%- if module.interfaces or has_handles %}
 import 'package:mojo/core.dart' as core;
 {%- endif %}
 {%- if should_gen_mojom_types %}
 {{mojom_types_import}}
 {%- endif %}
 {{service_describer_import}}
 
 {%- for import in imports %}
 import 'package:{{import.rebased_path}}.dart' as {{import.unique_name}};
 {%- endfor %}
 
 {%- include "module_definition.tmpl" %}