* Do not schedule animation if already at the target value

* Partially fixes https://github.com/flutter/flutter/issues/11495.
* Also includes a fix for Cupertino button to always run the tap animation even if the finger is immediately lifted from the screen (uncovered by a test failure).

* refactorings

* more tests

* test clearifications

* remove Listener

* fix lints

* fix async issue

Add Diagnosticable base class and documentation

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

Extract common text selection overlay logic from Material to Widget and create a Cupertino version of the overlays

Move some Cupertino colors to a common file. 

Create a CupertinoTabBar widget to mimic iOS looks

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

* Allowing overrides to the activeOpacity of the button

* adding tests

* fix

* fixing animation

* fixes

* moar fixes

The recommended minimum tappable area is actually 44.0pt according to the [iOS Human Interface Guidline](https://developer.apple.com/ios/human-interface-guidelines/visual-design/layout/).

> Provide ample spacing for interactive elements. Try to maintain a minimum tappable area of 44pt x 44pt for all controls.

I've also added an override to allow for flexibility in situations where designers get naughty and have a desire to make everything super small.

* Use exact Ahem and exact dimensions for test

* use ‘px’ everywhere

* Add cupertino to gallery and add CupertinoButto

* Use single quotes

* Add disabled state

* Some review notes

* Make button animation more responsive and tweak timing

* Renamed things Cupertino

* Button with background, move cupertino demos, move material demos

* Move 2 level list too

* Refactor various demo route names

* Some review notes

* More reviews and add test

* Linter as

* Move private constant up