This updates `AsyncSnapshot.data` to act as `AsyncSnapshot.requireData`
used to -- and it removes `AsyncSnapshot.requireData`. Correspondingly,
this adds a `StreamBuilder.withoutInitialData()` constructor, makes the
`initialData` argument to the default `StreamBuilder()` constructor required,
and deprecates the `initialData` argument to the `FutureBuilder()` constructor.

See the  breaking change announcement for more info.

https://github.com/flutter/flutter/issues/34545
https://groups.google.com/forum/#!topic/flutter-announce/H6Od0QdsdrI