Fixes #9989

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

* link more explicitly from route to navigator

* fix from review

This patch reworks some of the guts of scrolling to make it easier to
implement nested scrolling effects. The actually nested scrolling effect
will be included in a later patch.

* more widget const constructors

* prefer const constructors

* address review comments

Turns out we have many immutable classes.

Fixes #6892

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.

* add @required when there's an assert not null

* address review comments

Covers lib/ in package:flutter.

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.

* allow mulitple observers for Navigator and MaterialApp

* add test for the navigator observers

* fix style

* add test for adding/removing a navigator observer

Fixes for: https://github.com/flutter/flutter/issues/7570 https://github.com/flutter/flutter/issues/7231 https://github.com/flutter/flutter/issues/2841 and others

Also, synchronize the docs between Navigator.push and NavigatorState.push (and
friends).

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

Fixes #6963

Fixes https://github.com/flutter/flutter/issues/6451

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

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

These are late-breaking docs needed (mostly) for the iOS back gesture.

I did a pass through some of the code cleaning minor things up.

This eliminates all the issues around the horizontal page transition. I also improved the back gesture animation, and added fling support.

BUG=https://github.com/flutter/flutter/issues/5678
BUG=https://github.com/flutter/flutter/issues/5622

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.