Fixes#42950
It was previously allowed, though non-sensible, to read `.current`
before advancing the iterator for a `RegExp.allMatches` iterable and get
`null`. Using `as` allows backward compatibility in unsound null safety
mode, while `!` is breaking since it introduces a null check in either
mode.
The new behavior is also the behavior of the migrated code for DDC since
https://dart-review.googlesource.com/c/sdk/+/151121
Change-Id: I9f3654298a3d480ec2f297a40a4145ff6ca9f011
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157880
Auto-Submit: Nate Bosch <nbosch@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>
Nate Bosch
735bcbe10d