Create consistent copyright year across the DevTools codebase

Other codebases in Flutter use a consistent year in copyright headers on all source files. Some examples:

**flutter/flutter** - 2014 is the year
```
// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
```

**flutter/packages** - 2013 is the year
```
// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
```

We regularly have code review comments on PRs for the DevTools codebase to modify the copyright year to the current year or to revert a copyright year change for a moved file, and this causes excess churn on PRs and productivity loss. If we had a consistent year across the codebase, there would no longer be any debate, confusion, or errors on setting up a copyright header for a new file.