The way JS RegExps handle anchored matches is to have a separate RegExp
with `|()` added at the end, which means it either matches at that point,
or the `()` matches at that point, and that's visible in the captures
as a non-`null` last capture group.
The same RegExp is used to find the number of captures a RegExp has,
by running it on an empty string, and seeing the length of the capture
array, even if it contains nothing but `null` and `""` values.
The RegExp is as JS global regexp, which is what allows it to start matching
at a specific point, which we use for `startsWith(Pattern, start)`.
Every *normal* use of that RegExp remembers to set the `lastIndex` of the
regexp before using it, but the `regExpCaptureCount` used the "anchored
regexp" directly, and forgot to set the `lastIndex`, and since it had just
been used and failed to match at a position *later* than the current input
string's length, the "unfailable" test on the empty string failed, returning
`null` where no `null` was expected.
So now `regExpCaptureCount` sets `lastIndex` to zero before using the RegExp,
like every use of a shared RegExp should.
Fixes#56834
Bug: https://dartbug/com/56834
Change-Id: Ib649b70db5922c277950d7b7cfd4d157788d11cc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388002
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Lasse R.H. Nielsen
e260cc0330