* more widget const constructors

* prefer const constructors

* address review comments

* Manually fix every use of Point.x and Point.y

Some of these were moved to dx/dy, but not all.

* Manually convert uses of the old gradient API

* Remove old reference to Point.

* Mechanical changes

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\bPoint[.]origin\b/Offset.zero/g'
git ls-files -z | xargs -0 sed -i 's/\bPoint[.]lerp\b/Offset.lerp/g'
git ls-files -z | xargs -0 sed -i 's/\bnew Point\b/new Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bconst Point\b/const Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bstatic Point /static Offset /g'
git ls-files -z | xargs -0 sed -i 's/\bfinal Point /final Offset /g'
git ls-files -z | xargs -0 sed -i 's/^\( *\)Point /\1Offset /g'
git ls-files -z | xargs -0 sed -i 's/ui[.]Point\b/ui.Offset/g'
git ls-files -z | xargs -0 sed -i 's/(Point\b/(Offset/g'
git ls-files -z | xargs -0 sed -i 's/\([[{,]\) Point\b/\1 Offset/g'
git ls-files -z | xargs -0 sed -i 's/@required Point\b/@required Offset/g'
git ls-files -z | xargs -0 sed -i 's/<Point>/<Offset>/g'
git ls-files -z | xargs -0 sed -i 's/[.]toOffset()//g'
git ls-files -z | xargs -0 sed -i 's/[.]toPoint()//g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point, /show /g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point;/show Offset;/g'

* Mechanical changes - dartdocs

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\ba \[Point\]/an [Offset]/g'
git ls-files -z | xargs -0 sed -i 's/\[Point\]/[Offset]/g'

* Further improvements and a test

* Fix minor errors from rebasing...

* Roll engine

Nobody knew what a Block was.

This patch adds docs for many of the sliver widgets.

Also make locals final where possible.

Followup to 52715467.

* Improved defaults for scroll view primary-ness

* Vertical scroll views default to primary:true.
* Horizontal scroll views default to primary:false.
* If a scroll view is primary and it got a non-null inherited primary
  scroll controller, it introduces a primary scroll controller inherited
  with a value of null for its descendants.

ScrollController now multiplexes writes to all registered positions;
reads of position continue to assert that only one position is
registered. Reads still require a single position.

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.

We can't read our size in the offset setter because we might be in the middle
of layout. This issue also occured with the sliver-based viewport.

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.

There appears to be an issue with children before the center widget.
I've filled a bug about that issue and will follow up in a later patch.

This prepares us for a CustomScrollView that takes slivers.

This patch makes Scrollable2.ensureVisible with SingleChildScrollView. A future
patch will extend the implementation to work with slivers. (Although the patch
does include some of the infrastructure for that part of the implementation as
well.)

This property is wired up to anything and it isn't used. We'll need to find a
better way for clients to control the scroll offset.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

This widget is a replacement for ScrollableViewport that uses the new
Scrollable2 machinery. The widget is not based on Slivers but does use the new
scroll behavior classes.