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

Covers lib/ in package:flutter.

This functionality is independently useful.

Fixes #8293

This functionality got lots somehow in the migration to maybePop.

Fixes #8258

Move the back button and drawer opening logic into the app bar.

Move the tap-status-bar-to-scroll-to-top logic to using
ScrollControllers. Provide a PrimaryScrollController and a `primary`
flag on scroll views.

Make it possible to track when a route becomes or stops being poppable.

Previously the navigator wouldn't always call Route.dispose when it was
removed from the tree. After this patch, the navigator remembers popped
routes so that it can call dispose on them when it is removed from the
tree.

Also, improve some error messages around calling dispose() more than
once on routes and AnimationControllers.

Fixes #7457

We now create the Completer inside TransitionRoute, as with the popped
completer. Also, test that this future resolves at the appropriate time.

These futures complete when the route is popped off the navigator. This
generalizes and simplifies a mechanism already in place for dialogs and
menus.

Fixes #5283