Applies any additional bottom, left, and right media padding inside the
snackbar, if present. This accounts for the iPhone X home indicator
widget and horizontal padding for the sensor housing notch in landscape
orientation.

Now that keyboard height is modelled as a (bottom) view inset, migrate
scaffold bottom resizing to use view insets instead of bottom padding,
which, after an engine roll, will only be used for safe areas.

Until the aforementioned engine roll, the keyboard height is still
included in both bottom padding and view insets. As such
resizeToAvoidBottomPadding still drives bottom padding removal until
that roll lands.

Renames _ScaffoldLayout.bottomPadding to bottomViewInset

Also adds a TODO that should have landed with flutter/flutter#13423.

Scaffold bottom padding now applies the maximum of window
viewInsets.bottom (typically used for iOS safe areas) and padding.bottom
(typically used for keyboard height).

Also, extra docs.

Fixes https://github.com/flutter/flutter/issues/13279
Fixes https://github.com/flutter/flutter/issues/2115

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.

Adds `Scaffold#endDrawer` property to supply a second drawer to a Scaffold.

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.

* SafeArea

* AnimatedSafeArea

* AppBar test

* Apply feedback

Fix a bunch of obvious RTL bugs from code inspection.

Fixes #11369

* Notify the completer after the close animation completes on the snackbar

* Review comments

* Fix tests and analyzer warnings

* Fix analyzer warnings

It's redundant as iOS accessibility automatically includes this action on the clock in the status bar.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

With this patch, you can do:

```dart
   Future<Null> foo() async {
     try {
       await controller.forward().orCancel;
       await controller.reverse().orCancel;
       await controller.forward().orCancel;
     } on TickerCanceled {
       // did not complete
     }
   }
```

...in a State's async method, and so long as you dispose of the
controller properly in your dispose, you'll have a nice way of doing
animations in sequence without leaking the controller. try/finally
works as well, if you need to allocate resources and discard them when
canceled.

Simultaneously, you can do:

```dart
   Future<Null> foo() async {
     await controller.forward().orCancel;
     await controller.reverse().orCancel;
     await controller.forward().orCancel;
   }
```

...and have the same effect, where the method will just silently hang
(and get GC'ed) if the widget is disposed, without leaking anything,
if you don't need to catch the controller being killed.

And all this, without spurious errors for uncaught exceptions on
controllers.

* Scaffold appBar is-a PreferredSizeWidget, etc

* Updated

* Updated per review feedback

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

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