Update examples to adhere to new StreamBuilder API in Flutter 1.8.3+

[tvolkert]

No description provided.

[amirh]

It seems like the breaking change is potentially forcing packages to decide whether they want to support Flutter master or stable? left a comment on flutter/flutter#34626 (comment)

[collinjackson]

Once https://github.com/flutter/flutter/pull/36577/files is landed I think this PR can be closed without merging

[tvolkert]

Note the discussion in flutter/flutter#34545 (comment) and below. tldr: I still might not land flutter/flutter#36577. If it doesn't land, the way callers can be safe on both master and stable at the same time would be to explicitly pass initialData: null to the default StreamBuilder constructor.

[collinjackson]

Marking this PR as WIP since the new StreamBuilder API is still under discussion on flutter/flutter#34545, feel free to close or leave it open and ping us when it's ready for review.

[collinjackson]

We're closing this PR as part of the migration to a new repository but feel free to re-open it there once the new StreamBuilder API is finalized.