For example, so that the gallery can override the media query globally.

* Make FadeTransition more efficient

* Update fade_transition_test.dart

* Update proxy_box.dart

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

Fixes the remaining known issues with widgets supporting RTL.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Previously, we used `Alignment`, which was difficult to understand. Now,
we just use an `Offset` scaled to the child's size, which is much easier
to understand.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Fix a bunch of obvious RTL bugs from code inspection.

when describing non-nullable method arguments.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

Turns out that `[]:` means something in markdown, so you have to
escape the `:` with a backslash.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Fixes #6382

Covers lib/ in package:flutter.

Since RelativeRect's whole purpose in life is to make work with Stack
easier, it's silly that you can't directly use it with Positioned.

Also, tests for RelativeRect.
And fixes for the bugs that found...

We use `@protected` in this way in StatelessWidget to avoid developers calling
this function directly.

See #4434.

I did a pass through some of the code cleaning minor things up.

Fixes #4761

Also, add more dartdoc to widgets.dart.

Turns out we were storing the same object in the base class and the
subclass. For pretty much every AnimatedWidget subclass.

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint