Adding Copyright Headers

Formats

Dart Files (.dart)

// Copyright 20?? The Flutter Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.

YAML Files (.yaml)

# Copyright 20?? The Flutter Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.