Fix typo in unmodifiable map mixin return type.
Fix typo in unmodifiable map mixin return type.

Fixes #44264.

Bug: http://dartbug.com/44264
Change-Id: I958403af0e217cea6b45b4a74cb00fb800914f86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/173724
Reviewed-by: Kevin Moore <1d6e7d10d051cf619c5282da3c28fd8d3e0a65a4@google.com>
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@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