Extracted a superclass from AnimatedWidgetBaseState that is called ImplicitlyAnimatedWidgetState. ImplicitlyAnimatedWidgetState implicitly animates, but it does not call setState() automatically. Subclasses must respond to animation changes themselves. This is a useful optimization for Widgets whose render objects listen to animations directly. (#18173)

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

For example, mention the icon used for the drawer menu in the docs (this helps people writing unit tests); add DefaultAssetBundle sample code.

* Added sample code to AnimatedOpacity docs

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.

We've added a bunch of non-animatable properties to DefaultTextStyle.
This adds them to AnimatedDefaultTextStyle so that you can use them
even when your text style is animating.

UnconstrainedBox will allow its child to size itself as if it had no constraints, and then attempt to fit around that object, until its own constraints are exceeded, in which case it will clip and display an overflow warning.

I also factored out DebugOverflowIndicator, which will draw overflow indicators on containers which overflow but aren't expected to.

* Surface shadowColor from RenderPhysicalModel to Material

* Fix typo in material_test

* Add nullability info to documentation

* Add support for animating elevation shadow color

* Add shadowColor to Material's debugFillProperties()

* Add missing default value for elevation in Material debugFillProperties()

* Add missing non-null asserts for animate flags in AnimatedPhysicalModel

* Add test for shadow color animating smoothly

In order to allow chips to be properly drawn when they expand in size (without
using IntrinsicHeight), I needed a BoxDecoration shape that would be dependent
upon the rendered height of the widget. This seemed to be pretty generally
useful, so I added a new ShapeDecoration called StadiumBorder. It uses the
minimum dimension to adjust the BorderRadius of a rounded rect in the shape
decoration.

I also converted some uses of BoxShape to be case statements, updated the
chips to use the StadiumBorder decoration, and updated some of the metrics to match
the Material spec, as well as implementing lerping to and from StadiumBorder.

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.

* SafeArea

* AnimatedSafeArea

* AppBar test

* Apply feedback

This widget makes it easier to animated Positioned widgets with
awareness of the ambient Directionality.

Fixes #11998

Also, fix the interpolation between visual and directional fractional
offsets. The interpolation now works visually in whatever direction the
result eventually gets resolved into.

Fixes #11847
Fixes #11357

Add Diagnosticable base class and documentation

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

- How do you handle a tap on text?
- Why is AnimatedOpacity expensive?
- Why would you use a gesture arena team?
...and other minor fixes

* preview of prefer_asserts_in_initializer_list lint

* fix issue

Fixes https://github.com/flutter/flutter/issues/9342

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

* more widget const constructors

* prefer const constructors

* address review comments

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

* add @required when there's an assert not null

* address review comments

Covers lib/ in package:flutter.