* Add a callback that fires when a Draggable leaves a DragTarget.  This enables the DragTarget to manage its state from entry to exit.

* It helps to have a null check here

* Add test for onLeave callback and add verbiage to onWillAccept explaining the callback lifecycle of a DragTarget.

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.

* Add missing onDragCompleted to LongPressDraggable

* Add test for LongPressDraggable onDragStarted and onDragCompleted

Some widgets are using closures even if the only values that are
captured are this, context or widget, that can be accessed even from
methods of the State object.

Fixes #10350

* preview of prefer_asserts_in_initializer_list lint

* fix issue

* 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

* add @required when there's an assert not null

* address review comments

Covers lib/ in package:flutter.

...now that we have generic methods, their types need to be specified too.

More clearly explain the role of child, childWhenDragging, and feedback.

Fixes #6962

Fix _DragAvatar to display in the proper position for Overlays whose Point.origin is not the global Point.origin. (#7223)

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

This patch also makes it possible for a single DragTarget to handle
multiple draggables with the same data without asserting.

Fixes #6086

Fixes #6084

DragTarget would call setState after it was disposed in didLeave
and didDrop. Fixes #6128.

This structure for the API is hopefully less confusing that the previous one
(which implied that vertical drags would not trigger horizontally draggable
widgets).

Fixes #1987

In writing a test for #1927, I found a number of bugs in how Draggable shuts
down. Previously it would leak its recongizer. Now it disposes its recognizer
and the recognizer knows how to be disposed cleanly.

Fixes #1927

We have these details objects for the same reason we now have drag details
objects: future extensibility.

Also, add more dartdoc to widgets.dart.

I pulled the trigger to land the previous patch too quickly and missed a
number of comments.

These wrappers also give us a chance to document what these services do.

Fixes #3164

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

Previously the widgets layer only provided a concrete binding, which
makes it awkward to extend it compared to other bindings. This moves
widgets to the same style as the other layers.

In a subsequent patch I'll use this to make the tests layer saner.

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308