Adding CupertinoActionSheet, showCupertinoModalPopup

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

* Add doc example to addLocalHistoryEntry in LocalHistoryRoute (#9392)

This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.

The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.

Unsubscribe should remove RouteAware for all routes.

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

Previously, we used `Alignment`, which was difficult to understand. Now,
we just use an `Offset` scaled to the child's size, which is much easier
to understand.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Add Diagnosticable base class and documentation

...and break the navigator<->routes circular dependency.

This is a step towards fixing https://github.com/flutter/flutter/issues/9577 but doesn't yet do so.

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

Fixes https://github.com/flutter/flutter/issues/11323

* fix sample code

* review comments

* review comments

* document other members for extra bonus points

* Adds a PageRouteObserver

Required for https://github.com/flutter/plugins/pull/174.
See also https://github.com/flutter/flutter/issues/11216.

* formatting fixes

* Make RouteObserver generic

Previously, we would strand the old route in the overlay even though it had
been removed from the history.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

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

We now have an explicit focus tree that we manage. Instead of using
GlobalKeys to manage focus, we use FocusNode and FocusScopeNode objects.
The FocusNode is Listenable and notifies when its focus state changes.

Focus notifications trigger by tree mutations are now delayed by one
frame, which is necessary to handle certain complex tree mutations. In
the common case of focus changes being triggered by user input, the
focus notificiation still arives in the same frame.

The latter is the proper spelling.

Fixes #8883

The new name matches the style guide. (The old name was just old and predated
the style guide.)

Fixes #8000