Fixes https://github.com/flutter/flutter/issues/9342

This patch finishes documenting all the symbols except for those related to
scrolling.

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

* more widget const constructors

* prefer const constructors

* address review comments

* 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

Fixes https://github.com/flutter/flutter/pull/8643

Fixes https://github.com/flutter/flutter/issues/8804 by adding a test
to verify that Material elevation changes animate.

Also tweak some dartdocs, add some trailing commas, improve some
toString output, make the inkwell test agnostic to the PhysicalModel
logic, and move the material buttons test to the material test
directory.

Fixes https://github.com/flutter/flutter/issues/9186

Fixes #8519

Covers lib/ in package:flutter.

When a ListView scrolls, it generates a LayoutChangedNotification, which was
causing Material to repaint unconditionally. That's not necessary if there are
no ink effects that need to be moved. This patch skips the repaint in that
case.

Fixes #7937

Fixes https://github.com/flutter/flutter/issues/4620

This patch introduces ScrollView, which is a convenience widget for using a
SliverBlock. This patch also switches a number of tests from Block to
ScrollView. Once we support more features of block (e.g., padding and
shrinkwrapping), we'll be able to move over more clients.

Previously, these classes where in material.dart, which meant they could
access private interfaces in that library. This patch moves them out
into their own files so that they need to play by the rules of the
public API.

Fixes #5969

This should help a bit with people trying to make apps that have a
Material feel but don't necessarily exactly fit the Material spec.

Based on P5's experience

Previously we would maroon the feedback in the overlay. Now we let the
drag proceed and only tear down the gesture recognizer when all the
drags it spawns have been resolved.

Fixes #6151

Fixes https://github.com/flutter/flutter/issues/5763

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.

Using IconButton inside of a bigger Material before would result
in having a hit box as large as the whole Material. This commit
constrains the size of the hit box and splashes to the default
diameter of a splash.

Added a simple widget that automatically dispatches a
LayoutChangedNotification when its child changes layout.

* Added return value to a onNotification callback.

The LayoutChangedNotification callback was missing a return value.
This commit changes it to return true and stop notification from
bubbling up the tree.

* Changed _RenderInkFeatures to use fresh clip box.

Since it wasn't using the most current value of the RenderBox's
size, _RenderInkFeatures was rendering splashes incorrectly when
the underlying Material size was animating. This commit changes
the clip reference to use the size of the Renderbox instead of
the size method in order to get the newest value.

* Added custom radii to RRect.

This is the first commit towads an implementation of
MergeableMaterial. It adds custom radii to RRect.

* Renamed RRect constructors and added BorderRadius.

BorderRadius is a class similar to EdgeInsets that lets you define
all rounded corners of a rounded rectangle easily.

This patch starts working through the missing dartdocs in widgets.dart.

For consistency.

Fixes #4142

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

Making progress towards document all public APIs in material.dart. We're still
missing a few odds and ends (as well as missing docs in tabs.dart,
tooltip.dart, and two_level_list.dart).