* Add Colors.white38

* Update ThemeData.disabledColor and ButtonThemeData.disabledColor to Colors.white38

* Update pre-existing tests to expect Colors.white38 instead of Colors.white30

* Update API documentation to reflect these changes

* Implement ExpandIcon custom color, expandedColor, and disabledColor

* Update to use pumpAndSettle instead of hard-coded duration

* Update colors to unfocused state, added dark mode test to active state

* Fix Colors.white30 doc opacity value

* Add links to Material Design specifications to color, expandedColor and disabledColor

* Update API docs to reference dark theme material page

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Closes #3188

* rotate expansion icon fully if initial state expanded = true

* add test case for initially expanded

* remove unused import

* Update expand_icon_test.dart

add linebreak

* import dart:math as math

* more widget const constructors

* prefer const constructors

* address review comments

Changed the ExpandIcon constructor to take a default expansion state.
If the widget gets rebuilt with a different expansion value, the
animation will trigger but the callback will not.

Added a widget that automaitcally rotates the expansion icon and
offers a callback interface that captures presses.