* 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.
Now that protected can be accessed from inside the same library, we can
use protected in a number of new places and we can remove some
trampolines we were using to work around its previous semantics.
This patch starts using the mustCallSuper annotation for several of the key
lifecycle callbacks in the framework. I haven't added it to didUpdateConfig
because there are a large number of non-compliant overrides. We should discuss
whether we want to use it there.
