elevation property
final
      The elevation for the button's Material when the button is enabled but not pressed.
Defaults to 2.0. The value is always non-negative.
See also:
- highlightElevation, the default elevation.
 - hoverElevation, the elevation when a pointer is hovering over the button.
 - focusElevation, the elevation when the button is focused.
 - disabledElevation, the elevation when the button is disabled.
 
Implementation
final double elevation;