Fixes #632

Fixes https://github.com/dart-lang/stagehand/issues/632

* Update to Dart 2.5

* Bump version and update changelog

* Bump travis.yml

* Fixing tests

With the Dart Code extension: see https://dartcode.org/releases/v2-23/

* Mention in the contribution guide, that you might run into trouble if you are using a Windows maschine.
Add a .gitattributes file to force unix-style line endings on Windows maschines.

* small copy edit changes

Fixes #623 

Fixes #615.

Also update templates to Dart 2.4, except flutter_web_preview.

Add categories to --machine output

rev to 3.3.0-dev

* Test travis on oldest supported SDK, bump min SDK to 2.2.0.
* Add missing lint supported by dartfmt --fix
* Fix test

Closes #593.

Use ``` for examples - fixes https://github.com/dart-lang/stagehand/issues/577
Drop package name - fixes https://github.com/dart-lang/stagehand/issues/576

Fixes #571.

Require Dart 2 stable
web-angular: use stable dependencies

Fixes #568