DevTools License and 3P Notice Review

[mossmana]

• [x] Conduct a review of all code in DevTools and determine where licenses need to be added/updated and/or require a 3P notice for dependencies.
• [ ] Write a script that can add/update the licenses where necessary.

// Copyright 2019 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.

• [ ] DevTools should also show notices in an about dialog or help menu option.

e.g. Here's what the help menu option looks like in VS Code (just a link that redirects to https://code.visualstudio.com/license?lang=en)