The GetMapping calls removed in this patch had the same code and had to be repeated across different test harnesses as well as in dart_snapshot.cc. Just make this a factory method so the code is less verbose.
Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.
