This reverts commit 343b5703.

Reverting because the assert introduced in framework.dart(inheritFromElement) is breaking certain Mulligan pages. @hixie suspects it might be due to global keys.

Added InheritedModel:  An InheritedWidget that's intended to be used as the base class for models whose dependents may only depend on one part or "aspect" of the overall model.

* Visibility widget

This attempts to address the confusion around how to hide a widget subtree.

* Apply review comments

* More clarifications

RenderAndroidView is responsible for sizing and displaying an embedded
Android view.
AndroidView is responsible for creating and disposing the Android view
and is using RenderAndroidView to display it.

This adds a simple spacer widget first suggested by Scott Stoll (@ScottS2017) in the abandoned PR #15802

This PR replaces that one, cleans it up, and adds a test.

We don't really like the name AnimatedChildSwitcher, and we think that AnimatedSwitcher might be better (since the Child part is redundant). We also considered AnimatedChild, AnimatedTransition and AnimatedReplacement (among others).

Nothing in here besides a rename.

The AnimatedChildSwitcher widget (originally authored by Hixie), will cross-fade between a new child, and a previous child (or children, if the switch happens faster than the fade finishes).

It's a good candidate for places where a widget will be added/removed from a slot, and you want a nice transition to occur.

* Create ListWheelScrollView

* fix missing doc

* tweak docs a bit

* fix imports

* Add some tests

* review

* review and transform tests

* fix test

* repatch lost https://github.com/flutter/flutter/commit/977701cf695ec106a52dde9f3659a7b132826748

* review

* Review

* Create CupertinoScrollbar

* handle main axis margin

* Adaptive material scrollbar and tests

* Small tweaks

* reapply changes on head

* Docs

* start

* Refactored ScrollbarPainter to be more immutable

* fix tests

* fix bug: one animationcontroller pointed to multiple painters

* some docs tweak

* remove unused import

* review

* review

* add dispose

* SafeArea

* AnimatedSafeArea

* AppBar test

* Apply feedback

Bare bones widget inspector support.

Toggle the widget inspector from the flutter tool by pressing 'i'.
When the widget inspector is select mode:
Pointer down to to inspect a widget.
Pointer click to finalize selection of a widget. You can now interact
with the application as you normally would but with the inspected widget
highlighted.
Click the inspect icon in bottom left corner of screen to reactivate
select mode.

* FadeInImage: shows a placeholder while loading then fades in

* fix dartdoc quotes

* license headers; imports

* use ImageProvider; docs; constructors

* _resolveImage when placeholder changes

* address comments

* docs re ImageProvider changes; unsubscribe from placeholder

* rebase

* address comments

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments

Extract layout logic in material app bar to a common file that can be reused for cupertino

Move some Cupertino colors to a common file. 

Create a CupertinoTabBar widget to mimic iOS looks

Moved everything icons related except the material icon button and the materialicon font list to widget

Fixes https://github.com/flutter/flutter/issues/7988

A NestedScrollView lets you implement patterns where you have a header, tabs,
and additional scrollers inside the tabs.

Fixes #8359

This patch reworks some of the guts of scrolling to make it easier to
implement nested scrolling effects. The actually nested scrolling effect
will be included in a later patch.

Turns out we have many immutable classes.

Fixes #6892

* Scaffold appBar is-a PreferredSizeWidget, etc

* Updated

* Updated per review feedback

We now have an explicit focus tree that we manage. Instead of using
GlobalKeys to manage focus, we use FocusNode and FocusScopeNode objects.
The FocusNode is Listenable and notifies when its focus state changes.

Focus notifications trigger by tree mutations are now delayed by one
frame, which is necessary to handle certain complex tree mutations. In
the common case of focus changes being triggered by user input, the
focus notificiation still arives in the same frame.

The latter is the proper spelling.

Fixes #8883

This functionality is independently useful.

Fixes #8293

All the clients have migrated to Scrollable2.

- ScrollableList
 - ScrollableLazyList
 - LazyBlock
 - MaterialList

Clients should use ListView instead.

All the clients have migrated to GridView.  Also, remove RenderGrid,
which isn't needed by GridView.

Move the back button and drawer opening logic into the app bar.

Move the tap-status-bar-to-scroll-to-top logic to using
ScrollControllers. Provide a PrimaryScrollController and a `primary`
flag on scroll views.

Make it possible to track when a route becomes or stops being poppable.

All clients have moved to PageView.