* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

For example, mention the icon used for the drawer menu in the docs (this helps people writing unit tests); add DefaultAssetBundle sample code.

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

when describing non-nullable method arguments.

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

Also fixes a small typo.

* 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.

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

Add some commentary around DefaultTabController.initialIndex.
Fix typo (nuull) in TabBar.
Make some asserts more detailed.

Covers lib/ in package:flutter.

...now that we have generic methods, their types need to be specified too.