* 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.
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