* Reland "Nnbd widgets (#64672)"
This reverts commit 127e67902e.
* remove non-nullable enforcement on AsyncSnapshot
* make data param non-nullable for AsyncSnapshot.withData
* make some Text params non-nullable
* Reland "Nnbd widgets (#64672)"
This reverts commit 127e67902e.
* remove non-nullable enforcement on AsyncSnapshot
* make data param non-nullable for AsyncSnapshot.withData
* make some Text params non-nullable