update the pubspec file for pkg/nnbd_migration
update the pubspec file for pkg/nnbd_migration

Change-Id: I8ad96626dff34875394d4c5111549ae430de4ce5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/159145
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@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