Refactor "Format on Save" to route through Dart plugin

[stevemessick]

The code that implements "Format on Save" directly calls analyzer.edit_format(). It ought to use DartStyleAction.runDartftm(). The latter does more work to ensure file contents are updated.

[stevemessick]

I'd have to check, but I think this would fix #2847, too, if it is still an issue.