Mainly, this adds documentation to members that were previously
lacking documentation.

It also adds a big block of documentation about improving performance
of widgets.

This also removes some references to package:collection and adds
global setEquals and listEquals methods in foundation that we can use.
(setEquals in particular should be much faster than the
package:collection equivalent, though both should be faster as they
avoid allocating new objects.) All remaining references now qualify
the import so we know what our remaining dependencies are.

Also lots of code reordering in Flutter driver to make the code
consistent and apply the style guide more thoroughly.

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.

* preview of prefer_asserts_in_initializer_list lint

* fix issue

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

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

This feature lets you see a portion of the next and previous page in a
PageView.

Fixes #8408

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

This does not attempt to correct any logic, only to port it as written.

The API changed a bit to take into account what is newly available and
no longer available in the new world.

Fixes https://github.com/flutter/flutter/issues/8017

Also, rename ScrollableMetrics to ScrollMetrics, which follows the
naming convention for most of the other classes (e.g., ScrollPosition,
ScrollPhysics).

Finally, fix a bug whereby SingleChildScrollView could not have a
GlobalKey, because, write test, find bug.

This patch improves PageView to the point where we can use it in the date
picker. Specifically, you now get onPageChanged notifications and you can
control which page is visible using a PageController.