Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments

Applies comments I missed from https://github.com/flutter/flutter/pull/10904.

I got a bit confused by these comments when trying to debug this code,
let's try clearing them up a little.

* Revert "Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)"

This reverts commit ceb814aa.

Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)

This reverts commit d46e208b.

* Change all instances of '$runtimeType#$hashCode' to use the describeIdentity method.

The describeIdentity method generates a shorter description with a consistent length
consisting of the runtime type and the a 5 hex character long truncated
version of the hash code.

Minor fixes throughout, e.g. removing trailing commas from the end of sample code expressions, changing headings to "sample code" more consistently, removing stale todos, fix typos in a private method name, minor grammar fixes, added some clarifications to CircularProgressIndicator, LinearProgressIndicator, CrossAxisAlignment, added some cross-references to various members, made it slightly clearer that layout algorithms are implementation details.

Clarified "elevation" throughout.

Added docs to InkResponse and InkWell.

Added sample code for: SliverAppBar, Card, ListTile, EdgeInsets, Row, Column, CustomScrollView, ListView, SliverFixedExtentList, and SliverGrid.

Fixes #10317.
Fixes #10316.
Fixes #10267. (sort of, see comment therein)
Fixes #9331. (sort of, see comment therein)
Fixes #9407. (sort of, see comment therein)

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

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

We now attempt to explain what went wrong.

Fixes #9509

This patch adds docs for many of the sliver widgets.

This patch adds a notification to SliverChildDelegate that says which
children are alive after each layout. The delegate can use this
information to optimize it's underlying model of the children (e.g., by
discarding models for children that are far outside the live range).

Fixes #9045

Covers lib/ in package:flutter.

This feature lets you see a portion of the next and previous page in a
PageView.

Fixes #8408

Fixes #7734

* More better toStrings and more vigorous smoke testing of gallery

* Update scroll_controller.dart

* Update sliver.dart

We weren't triggering a relayout when the number of children changed.

Fixes #8001

After this patch, the old grid code is not used in the framework.

This patch uses sliver-based two more gallery demos, the stocks example,
in the date picker, and in markdown.

We now create a GridLayout object that memoizes the layout information
for a given SliverConstraints. This approach is both more efficient
(because the delegate no longer needs to re-solve its layout for each
child) and allows for more code sharing between delegates.

The SliverGridRegularTileLayout will also be useful for the date
picker.

Also, document the grid delegates.

This patch moves the resonsibility for wrapping repaint boundaries
around children to SliverChildDelegate, which means delegates can choose
whether or no to use repaint boundaries.

Also introduce SliverChildBuilderDelegate to make it easier to use the
builder pattern with sliver lists.

This functionality will be used by date picker, which wants to use a
SliverGrid but doesn't need a repaint boundary around every day in a
month grid.

This widget is a start towards replacing PageableList. There are still a number
of features that we'll need to add before this widget can replace PageableList.

SliverFill fills the remaining space in the viewport with each box
child. We'll use this sliver as a building block for pageable lists.

This patch adds grid supports to slivers and introduces a ScrollGrid
convenience class for making the common types of scrollable grids.

This patch also deploys ScrollGrid in an example in the Flutter Gallery.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.

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

Uses SliverPadding to implementing paddding in ScrollView. Also, deploy
ScrollView in more places now that it implements padding.

Finally, remove loader_app.dart because it is not referenced.