Fixes #19523

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

* controller, position and test

* Make controllers swappable

* WIP

* Create a ListWheelScrollPhysics

* Created picker and gallery demo and testing now

* Works. Ready to document and test.

* Document and add tests. Make the scroll controller more generic.

* minor cleanup

* review

* review

* fix tests

* stop using TransformLayers for now

* partial

* add doc and warning

* add correction

* review

* fix test

* Add a pageSnapping parameter to PageView

Setting the pageSnapping property allows extending the PageView scroll
behavior, such as custom scroll animations or custom scroll bars.

* Apply pageSnapping CR feedback

- Remove _kNonSnappingPhysics, use null instead.
- Minor code style fixes.
- It turns out that the forth state is Arkansas, not California.

Change PageController#nextPage & #previousPage methods to return Future when the animation completes. (#12299)

* Changed PageController #nextPage & #previousPage to return a Future after completing.

* Removed false statement about returning a Future in PageController#jumpToPage

* Added tests to make sure nextPage & previousPage return Futures.

* Tested that the Futures returned by nextPage & previousPage actually resolve.

* Renaming *pageCalled to *pageCompleted, since that's what it's actually for.

Fixes #11855

This patch includes:

 * SliverPadding
 * ScrollView
 * PageView

Fixes #11764
Fixes #11836

Add Diagnosticable base class and documentation

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

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 reverts commit c8e4cbf2.

* preview of prefer_asserts_in_initializer_list lint

* fix issue

It wasn't tested, which is why it didn't work before.

* Adjust the defaults behaviour of scroll views.

Now, primary scroll views scroll by default. Others only scroll if necessary.

* apply suggested changes

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.

This constructor makes it easier to use a GridView with a builder
callback.

Fixes #8952

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

Mostly related to semantics and scrolling.

* clamp the pixels used in the page calculation to be within min + max extents to avoid pages outside of the range of children

* add test to check onPageChanged not fired on overscroll

Covers lib/ in package:flutter.

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

Fixes #8408