[parser] Dot shorthands: Recover as a DotShorthandPropertyAccess rather than a PrefixedIdentifier.

This change mainly has side effects for code completion, making sure we're suggesting the right members. When the user writes something like `E e = .^` where `^` is the cursor, they're most likely intending to write a dot shorthand.

So this parser recovery change ensures that we recover as a dot shorthand node and not as a prefixed identifier (missing it's target) in the analyzer. We don't need special casing in the code completion pass to handle dot shorthands in the prefixed identifier visitor now.

The CFE should have no notable changes, but I added a few tests since it uncovered some crashing behavior. The duplicate errors have always been the case even prior to this change.

Bug: https://github.com/dart-lang/sdk/issues/59836
Change-Id: I6c4a9f2c8cc376e4dedd715af76967da471aa681
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/439140
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>