* Introduce a Directionality inherited widget which sets the ambient LTR vs RTL mode (defaulting to null, which means you cannot use directionality-influenced values).

* Make it possible to configure Padding (including Container.padding and Container.margin) using a directionality-agnostic EdgeInsets variant.

* Provide textDirection and verticalDirection controls on Row and Column to make them RTL-aware.

* Introduce a variant of FractionalOffset based on the EdgeInsets variant. Not yet actually used.

* Fix all the tests that depended on Row defaulting to LTR.

Aggressively apply the const lint.

* prefer const constructors

* fix double throw

Extend app bar left to edge, right to 4dp
Make leading button square and 56dp
Keep title at 72dp on Android according to Material
Renamed IconButton.size to .iconSize
IconButton minimum size expands to 48dp (#8264)
IconButton default constraints to 48. Can still stretch to infinity but can't be smaller than 48.
Ink splash for IconButton 40% bigger than the touch target to match Material
Tests

Fixes https://github.com/flutter/flutter/issues/5763

Using IconButton inside of a bigger Material before would result
in having a hit box as large as the whole Material. This commit
constrains the size of the hit box and splashes to the default
diameter of a splash.