This CL deprecates the front-end API and moves it into src/api_prototype.
For now all usages have been updated to point to the new location,
but they should be updated to use custom-client invocations instead (e.g.
one specific way for DDC, another for dart2js etc.)
Bug:
Change-Id: I9b4f41f6ebf55d42510fd35240d942d1dc7292d6
Reviewed-on: https://dart-review.googlesource.com/24822
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Kevin Millikin <kmillikin@google.com>