This fixes a problem where a defaulted optional argument
would cause the passed type parameters to be replace with dynamic.
Change-Id: I32cf327b3a4b6176f199164c840a1114fc302205
Reviewed-on: https://dart-review.googlesource.com/49081
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>