Covers lib/ in package:flutter.

Fixes #8413

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.

Closes https://github.com/flutter/flutter/issues/6907

The docs now include all the information contained in the exception.

Fixes #6994

Fixes #6915

Fixes #6963

People get confused by the tight constraints.

Fixes https://github.com/flutter/flutter/issues/6628

Closes https://github.com/flutter/flutter/issues/5781

TBR @abarth

...until people understand them.

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

Fixes #616

Developers need to get the size of the BuildContext sufficiently often
that we should provide a convenient getter for the value. Having this
getter is also an opportunity to catch common mistakes and provide
useful error messages that guide developers towards better patterns.

Fixes #2321

The spec forbids persistent bottom sheets from overlapping the app bar. There
are also some fancy scroll-linked effects that we're supposed to do with
persistent bottom sheets, but those will need to wait for another patch.

Fixes #5143

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.

* Added OverscrollIndicatorEdge et al

* RefreshIndicator only clamps its scrollable edge

* added a test

* Updated the test

* fixed lint-os

* fixed a typo

* Scrollable should restore its viewport dimensions when it reappears

* removed an accidental commit

* updated per review feedback

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

The iOS transition is the more buggy part of this feature, and it wasn't
disabled. Let's just leave them enabled while I work on improvements.

Also added a regression test for the back gesture on iOS and android.

Doesn't do any of the fancy effects. Just lets the user control the
back transition by sliding from the left, like a drawer. Hero
transitions are disabled during the gesture.

BUG=https://github.com/flutter/flutter/issues/4817

Fixes https://github.com/flutter/flutter/issues/5131

If the scaffold is new and has a floating action button, we skip the entrance
animation for the fab.

Fixes #4445