Fix parsing of `PATTERN as T? when`

The logic for parsing types has special disambiguation rules for
deciding whether a trailing `?` should be included in the type, based
on what token(s) follow the `?`.  In the case where the token that
follows the `?` is `when`, we need to look further ahead to
disambiguate, to distinguish `PATTERN as T? when guard` from something
like `EXPRESSION is T ? when : otherwise`.

(Note: an alternative implementation would be to disambiguate based on
whether we're parsing a pattern or an expression.  But in the future I
want to move toward an architecture where expression parsing and
pattern parsing are combined, so that if the parser makes the wrong
decision about whether it's looking at a pattern or an expression,
error recovery will do a better job.  So I'm disambiguating based
solely on what follows the `?`.)

Bug: https://github.com/dart-lang/sdk/issues/50035
Change-Id: Idbc780b7b54fecc7fd01cae868c34771564dd804
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/292282
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>