When non-nullability was made the default, it was then desirable to have the
most commonly used APIs return a non-nullable value. This is because saying
MediaQuery.of(context, nullOk: false) and then still requiring an ! operator
or ? and a fallback value after that call felt awkward.

The nullOk parameter was a cheap form of providing a null safety toggle, which
in the face of true language support for non-nullability, was then supplying
redundant, and perhaps contradictory signals to the developer.

To solve this, the of accessors (and some related accessors that also used
nullOk) were split into two calls: one that returned a non-nullable value and
threw an exception when the sought-after widget was not present, and one that
returned a nullable value that didn’t throw an exception, and returned null if
the widget was not present.

The design document for this change is Eliminating nullOk parameters.

Description of change