"View source code" shouldn't point to master branch

[Piinks]

"View source code" button on the api reference page currently points to the master revision of source code. It also highlights wrong line number, because it was deployed when the code was different.

Example:

1. Go here
2. Click "View source code" (document icon)
3. You're correctly redirected to form.dart file, but on master branch.
4. Tho highlighted line is 776, while the correct code is at 807.

I'm proposing that this button and the version tag displayed at the bottom should redirect to the stable version of Flutter's source code.

Originally reported in flutter/flutter#169065 by @xVemu