Context

Rather than try and evolve the existing button classes and their theme
in-place, we have introduced new replacement button widgets and
themes. In addition to freeing us from the backwards compatibility
labyrinthe that evolving the existing classes in-place would entail,
the new names sync Flutter back up with the Material Design spec,
which uses the new names for the button components.

FlatButton

ButtonTheme

TextButton

TextButtonTheme

RaisedButton

ButtonTheme

ElevatedButton

ElevatedButtonTheme

OutlineButton

ButtonTheme

OutlinedButton

OutlinedButtonTheme