- Fixed extentInside calculation in ScrollMetrics
- Added asserts to extentInside getter, as well as ScrollPosition.applyContentDimensions to enforce minScrollExtent <= maxScrollExtent
- Added padding to ScrollbarPainter, updated implementation. Took care of some edge cases.
- Changed some scroll bar constants on Cupertino side.

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Fixes #19523

New a11y traversal:

- sort direct sibling SemanticsNodes only
- use new sorting algorithm
- implement RTL
- test semantics in traversal order by default
- add AppBar traversal test
- breaking: remove nextNodeId/previousNodeId from the framework
- breaking: remove DebugSemanticsDumpOrder.geometricOrder

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* PageView ballistics overshoot the page on some devices

On some devices, such as Cupertino “Plus”-sized devices, scrolling left on the first page of a PageView will overshoot the first page and land on the second page.

The issue is that applyContentDimensions incorrectly detects a content size change due to a floating point comparison on certain screen sizes (18257.400000000005 vs 18257.4)

To fix this, perform a nearEqual comparison in applyContentDimensions.

* Apply style changes to nearEqual for code review feedback.

* fix miscellaneous typos in dartdocs

* moar typos

* Update Semantics for SingleChildScrollViews

* refactor

* review feedback

* added assert and comments

* doc

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.

* a11y: implement new SemanticsAction "showOnScreen" (v2)

This action is triggered when the user swipes (in accessibility mode) to the last visible item of a scrollable list to bring that item fully on screen.

This requires engine rolled to flutter/engine#3856.

I am in the process of adding tests, but I'd like to get early feedback to see if this approach is OK.

* fix null check

* review comments

* review comments

* Add test

* fix analyzer warning

* Expose the currently available semantic scroll actions

* review comments

* add test

* refactor to set

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.

Fixes https://github.com/flutter/flutter/issues/8476

More detailed list of changes in this patch:

* Replaced the didTouch special logic with more generic logic that
  uses Activities instead. Now instead when you tap down the
  Scrollable calls `hold()` which begins a HoldScrollActivity which is
  a hybrid of DragStartDetails and IdleScrollActivity and can be
  canceled. When you let go, it gets canceled and that goes ballistic.

* Make DragGestureRecognizer more aggressive about grabbing pointers,
  otherwise a second pointer in a situation with competing horizontal
  and vertical recognizers always gets taken by the other one.

* Fixed the _GestureSemantics widget to call the "down" callbacks so
  that it follows the same pattern as "real" interactions.

* Added tests for the above.

* Added a hashCode to ScrollActivity.toString (and subclasses).

* Added a toString to ScrollDragController, and include it in
  DragScrollActivity's toString.

* s/coorindator/coordinator/

* Add a comment in DragStartDetails to distinguish it from the
  otherwise identical DragDownDetails, so we're not tempted to merge
  them.

Previously, ScrollPosition did not know about ScrollActivities. However, all
the concrete subclasses of ScrollPosition that we know about need to use
ScrollActivities, so they ended up with a bunch of delegate boilerplate code.

This patch teaches ScrollPosition about ScrollActivities but doesn't have any
opinion about how to start or interact with those activities.

This patch is more refactoring to prepare for nested and linked scrolling.