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.

This is a collection of fixes I've run into recently.

Fixes https://github.com/flutter/flutter/issues/10695

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

Separate focus management by build owner

Currently the focus manager is held by the singleton WidgetBinding.
This change places a focus manager in each build owner object,
which allows clients to run offscreen pipelines without disturbing
focus management for the main UI.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* custom hashCode/== in Element to speed up inheritFromWidgetOfExactType

* explain why 24 bits

Various improvements (in particular a new painting fuzzer) to the text manual test.

Some additional documentation.

A fix to Stack to remove an LTR bias: make unpositioned children apply "alignment".

Some more debugging information on RichText and Text.

A fix to the flutter tool to not crash when an RPC call throws an exception.

* fix miscellaneous typos in dartdocs

* moar typos

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._

* enable lint prefer_foreach

* fix tests

* Revert "Construct the accessibility channel's events by SemanticsEvent. (#12638)"

This reverts commit 4c1150dd.

* Revert "Revert "Add a way to get the furthest Navigator" (#12608)"

This reverts commit 79f13c36.

* Revert "BoxDecoration.borderRadius and RoundedRectangleBorder.borderRadius RTL (#12603)"

This reverts commit 07908916.

* Revert "Copy snapshot file into assets (#12523)"

This reverts commit f6135107.

* Revert "Add a rootNavigator option to Navigator.of (#12580)"

This reverts commit 822084b2.

* Update Semantics for SingleChildScrollViews

* refactor

* review feedback

* added assert and comments

* doc

* Revert "When parts of the program are changed in a hot reload, but not executed during the reassemble, warn that a restart may be needed. (#12304)"

This reverts commit 90028813.

* fix _debugCheckOwnerBuildTargetExists; sync localizations and tests

* address comments

* optimize ThemeData: make it monomorphic, memoize result

* address comments

* RLU cache; fix text theme merging

* use FIFO cache for ThemeData; use HashMap to store inherited widgets

* address comments

* Fix inherited widget notifications in Localizations

* address comments

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.