Diagnostics object is.

* rename Element.visitChildrenForSemantics to debugVisitOnstageChildren

* address comments

* fix typo in State.didUpdateWidget

* rephrase

In some cases, the element tree is not clean but we are required to dump the tree anyway.
To avoid crashing in those cases, we return an explicit null node.

Add Diagnosticable base class and documentation

It was 8.0. It's now arbitrarily 18.0.

Changing this required adjusting some tests. Adjusting the tests
required debugging the tests. Debugging the tests required some tools
to help debugging gesture recognizers and gesture arenas, so I added
some. It also required updating some toString() methods which resulted
in some changes to the tree diagnostics logic.

Also I cleaned up some docs while I was at it.

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

Mainly, this adds documentation to members that were previously
lacking documentation.

It also adds a big block of documentation about improving performance
of widgets.

This also removes some references to package:collection and adds
global setEquals and listEquals methods in foundation that we can use.
(setEquals in particular should be much faster than the
package:collection equivalent, though both should be faster as they
avoid allocating new objects.) All remaining references now qualify
the import so we know what our remaining dependencies are.

Also lots of code reordering in Flutter driver to make the code
consistent and apply the style guide more thoroughly.

* 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

* 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.

This reverts commit c8e4cbf2.

* preview of prefer_asserts_in_initializer_list lint

* fix issue

Replace a misplaced StatelessWidget class with StatefulWidget class

I added a bunch of sample code to the core Widget subclasses so that
people would see that you are supposed to always include the `key`
argument.

We now attempt to explain what went wrong.

Fixes #9509

* comprehensive list of lints
* add comments to commented out lint
* fix unnecessary_this lints
* exclude prefer_final_fields

Turns out we have many immutable classes.

Fixes #6892

There are more places we can use this annotation, but this patch just gets us
started.

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

Also, add a specific message for BuildContext.size for RenderSlivers.

Still many more to go.

This yak shave went as follows:

Fix https://github.com/flutter/flutter/issues/8795 by adding stocks to
the examples README.

Notice the layers entry in that README isn't quite right either.
Update that.

Check the layers/README file is worth pointing at.

Update the layers/README.

Let's run some of the layer tests to see if they still work.

Oops, need to update them to gradle.

Ok let's try running them again.

Oops, sector is broken.

Add a test for sector.

Fix sector. Find you need to add an assert to a const constructor.

Notice we need to turn const asserts on for the analyzer.

Notice the analysis_options files are out of sync with each other and
with the full list of lints.

Turn on the lints that should be on.

Fix the bugs that finds.