A lot of this migration was performed when we eliminated nullOk parameters in
a previous migration, but some of methods were missed in that migration,
and some were subsequently added with nullable return types, counter to our
common pattern.

In this migration, the affected of accessors were split into two calls: one
that returned a non-nullable value and threw an exception when the sought-after
value was not present (still called of), and one that returned a nullable
value that didn’t throw an exception, and returned null if the value was not
present (a new method called maybeOf).

Description of change

The change modified these static of APIs to return non-nullable values. If a
value is not found, they will also now assert in debug mode, and throw an
exception in release mode.

AutofillGroup.of

DefaultTabController.of

DefaultTextHeightBehavior.of

Form.of

HeroControllerScope.of

Material.of