Updating iterator expected values from null to -1.
Updating iterator expected values from null to -1.

Change-Id: Ic143c6382a971a6b3408180ec8c5c56077b04f5c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132406
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Commit-Queue: Mark Zhou <654f5fe0c4ca6c799b0e735dcf3fc1c389f35bf9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk