* migrates lib/foundation and lib/widgets

* migrates lib/painting and lib/material

* migrates lib/cupertino, lib/gestures, lib/services, lib/semantics and lib/rendering

* fixes failing tests

* migrates test/

* improves hashing of lists

DataTable and Button NNBD docs fixes

My yearly pass to fix spelling errors in the API dartdocs.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

This reverts commit 81439922.

* migrate some material files to nullsafety

* review