Update DEPS for package:fixnum
Update DEPS for package:fixnum

Update in advance of nnbd work on fixnum for accurate attribution of
benchmarking changes.

Change-Id: I69bad3bb21c16bed149e4c30ac82a351bc6db203
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149846
Commit-Queue: Kevin Moore <1d6e7d10d051cf619c5282da3c28fd8d3e0a65a4@google.com>
Auto-Submit: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>
Reviewed-by: Kevin Moore <1d6e7d10d051cf619c5282da3c28fd8d3e0a65a4@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