This PR adds the linux and windows target platform enum values, along with automatically setting the defaultTargetPlatform to the appropriate value on those platforms.
Fixes#31366
This implements VisualDensity changes for text fields*. By default, the layout of the text field does not change.
If the ThemeData.visualDensity is set to a value other than zero, then the density of the UI will increase or decrease. See the VisualDensity docs for more information.
(*In reality, the changes are on the InputDecorator class, not on the text field.)
I also fixed a problem that I think I found with _Decoration where it doesn't compare isDense or isCollapsed as part of its operator==.
Adds a manual test that allows testing of density for buttons. Also updates some of the button tests to be somewhat simpler and to test the child positions in the test to make sure they are consistent.
