This updates the `ApplyCodeActionCommandHandler` to be a "shared" command that can work for either kind of server (although it does not yet allow calling over DTD - the executeCommand handler still requires trusted callers).
It also moves all of the tests into a shared mixin so they will be run for both server kinds.
Change-Id: I831cc5a1a9feadc528e5b33b82e3469c60111dfa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427440
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Danny Tuppeny