Drag from the right is no more
Longpress is now only 100ms instead of 500
Added optional duration field to longpressgesturerecognizer
Added controller field to material scrollbar api
Haptic feedback only triggers when scrollbar is fully expanded
Added haptic feedback when releasing the scrollbar after dragging it

- 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.

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

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.

* Create CupertinoScrollbar

* handle main axis margin

* Adaptive material scrollbar and tests

* Small tweaks

* reapply changes on head

* Docs

* start

* Refactored ScrollbarPainter to be more immutable

* fix tests

* fix bug: one animationcontroller pointed to multiple painters

* some docs tweak

* remove unused import

* review

* review

* add dispose

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

Summary:

- Add `key` field to `SemanticsNode`, while moving key into `foundation` library so it can be used by the render layer.
- Introduce `SemanticsProperties` and move many of the `Semantics` fields into it.
- Introduce `CustomPaintSemantics` - a `SemanticsNode` prototype created by `CustomPainter`.
- Introduce `semanticsBuilder` and `shouldRebuildSemantics` in `CustomerPainter`

**Breaking change**

The default `Semantics` constructor becomes non-const (due to https://github.com/dart-lang/sdk/issues/20962). However, a new `const Semantics.fromProperties` is added that still allowed creating constant `Semantics` widgets ([mailing list announcement](https://groups.google.com/forum/#!topic/flutter-dev/KQXBl2_1sws)).

Fixes https://github.com/flutter/flutter/issues/11791
Fixes https://github.com/flutter/flutter/issues/1666

When scrolling vertically, the scrollbar is on the left in RTL.

Fixes https://github.com/flutter/flutter/issues/7648

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.

* make @immutable const

* fix build

* 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

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

The new name matches the style guide. (The old name was just old and predated
the style guide.)

Fixes #8000

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.

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 makes a number of changes to how you can configure a
Scrollable2:

 - The ScrollPhysics is now responsible for creating the ScrollPosition.
   You can override the ScrollPhysics by supplying a `physics` argument
   to `Scrollable`, and the new physics you supply will be applied to
   the default physics inherited from the ScrollBehavior.

 - This patch removes the ScrollPosition/AbsoluteScrollPosition split as
   all clients were operating in pixels anyway and the split made the
   code very difficult to follow.

 - ScrollPosition no longer depends directly on Scrollable2State.
   Instead, it depends on an abstract interface that Scrollable2State
   implements. This change has two benefits:

    a) It removes the circular dependency between ScrollPosition and
       Scrollable2State, which lets us split the code for these classes
       (and several other classes that got wrapped up in that cycle) into
       separate libraries for easier maintenance.

    b) ScrollPosition is no longer bound to Scrollable2, which means you
       could use the behavior machinery to drive other sorts of widgets.
       For example, we could use it to drive Scrollabe1 if we wanted.

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.

Fixes #5393.

And make Scrollbar work with LazyBlock.

And an about box to the Stocks sample app.

This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.

* Added Scrollbar, removed ScrollbarPainter

* removed a dead import

* updated per review feedback

* Only call dispatch if the widgets State is still mounted