Update source_class.dart

Author: davidmorgan

built_json_generator/lib/src/source_class.dart

@@ -1,3 +1,7 @@
+// Copyright (c) 2015, Google Inc. Please see the AUTHORS file for details.
+// All rights reserved. Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 library built_json_generator.source_class;
 
 import 'package:analyzer/src/generated/element.dart';