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

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