This prevents some of our mixins from being subclassed.

Also, move mixins to using 'extends' instead of 'implements' for
future compatibility with Dart changes.

Also, rename a class that had Mixin in the name but was not a mixin.

Minor fixes throughout, e.g. removing trailing commas from the end of sample code expressions, changing headings to "sample code" more consistently, removing stale todos, fix typos in a private method name, minor grammar fixes, added some clarifications to CircularProgressIndicator, LinearProgressIndicator, CrossAxisAlignment, added some cross-references to various members, made it slightly clearer that layout algorithms are implementation details.

Clarified "elevation" throughout.

Added docs to InkResponse and InkWell.

Added sample code for: SliverAppBar, Card, ListTile, EdgeInsets, Row, Column, CustomScrollView, ListView, SliverFixedExtentList, and SliverGrid.

Fixes #10317.
Fixes #10316.
Fixes #10267. (sort of, see comment therein)
Fixes #9331. (sort of, see comment therein)
Fixes #9407. (sort of, see comment therein)

* comprehensive list of lints
* add comments to commented out lint
* fix unnecessary_this lints
* exclude prefer_final_fields

Turns out we have many immutable classes.

Fixes #6892

With this patch, you can do:

```dart
   Future<Null> foo() async {
     try {
       await controller.forward().orCancel;
       await controller.reverse().orCancel;
       await controller.forward().orCancel;
     } on TickerCanceled {
       // did not complete
     }
   }
```

...in a State's async method, and so long as you dispose of the
controller properly in your dispose, you'll have a nice way of doing
animations in sequence without leaking the controller. try/finally
works as well, if you need to allocate resources and discard them when
canceled.

Simultaneously, you can do:

```dart
   Future<Null> foo() async {
     await controller.forward().orCancel;
     await controller.reverse().orCancel;
     await controller.forward().orCancel;
   }
```

...and have the same effect, where the method will just silently hang
(and get GC'ed) if the widget is disposed, without leaking anything,
if you don't need to catch the controller being killed.

And all this, without spurious errors for uncaught exceptions on
controllers.

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

* address review comments

Covers lib/ in package:flutter.

This patch aligns the iteration patterns used by animations and
ChangeNotifier. They now both respect re-entrant removal of listeners
and coalesce duplication registrations. (Also, ChangeNotifier
notification is no longer N^2).

This patch introduces ObserverList to avoid the performance regression that the
previous version of this patch caused.

Fixes #7533

This patch aligns the iteration patterns used by animations and
ChangeNotifier. They now both respect re-entrant removal of listeners
and coalesce duplication registrations. (Also, ChangeNotifier
notification is no longer N^2).

Fixes #7533

Previously the navigator wouldn't always call Route.dispose when it was
removed from the tree. After this patch, the navigator remembers popped
routes so that it can call dispose on them when it is removed from the
tree.

Also, improve some error messages around calling dispose() more than
once on routes and AnimationControllers.

Fixes #7457

The only client of this machinery was AnimationController.fling, but
it's easier for that function to just create the simulation it needs
directly.

Fixes #7216

Port DecelerateInterpolator to Flutter.

Also, hide the Curve subclasses that you can't extend and that have no
statics and are generally not useful.

(and minor doc changes)

This will in the future allow Scrollable to not track the Simulation itself.

For consistency with Tween.

Fixes #5169

* globalToLocal was just broken when there was a rotation and a
  translation at the same time. This fixes that and adds a test.

* update graphic used by spinning_mixed since the old one went 404.

* simplify some of the code in the demo.

* fix MatrixUtils.transformPoint to be consistent with how we transform
  points elsewhere.

* stop transforming points elsewhere, just use
  MatrixUtils.transformPoint.

* make the Widget binding handle not having a root element.

* make the spinning_mixed demo update its widget tree.

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

See #4434.

Having this base class lets classes like CustomPainter and DataTableSource be
more agnostic as to what's generating the repaints.

In the interest of using the name in the case of Material steppers,
this commit renames Step curve to Threshold.

Doesn't do any of the fancy effects. Just lets the user control the
back transition by sliding from the left, like a drawer. Hero
transitions are disabled during the gesture.

BUG=https://github.com/flutter/flutter/issues/4817

This also required changing the AnimationController state transition
logic to signal completion of the animation during the tick that
finishes the simulation.

Fixes https://github.com/flutter/flutter/issues/3675

This patch adapts the AppBar to feel more like iOS by centering the title.

Fixes #4962