These arguments are actually required because we assert that they're not null.

Overflow handling works with clipping, adding an ellipsis on the last
line, or fading the last line.

Fixes https://github.com/flutter/flutter/issues/7271

For consistency with IconTheme and TextStyle.merge.

Fixes #4652

This feature is the implementation of an accessibility feature that changes the
size of text by a constant factor.

Fixes #5873