If the client supports Interactive Forms, this will allow prompting for a name for the import prefix instead of using "prefix" (or "prefix1", etc.).
Includes moving some boilerplate out of each refactor into `ParameterizedRefactoringProducer` and support for a custom validation function for form fields (to validate the import prefix name in this case).
Change-Id: I7150664a18944c723eeeac0a309341af2860201f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509340
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Danny Tuppeny
8f30cb7e43