This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

In the old world, we had two ways to bind a Widget tree to a
RenderObject node, one way for RenderView and one mostly untested way
for other cases (it's only tested by the spinning_mixed.dart demo). For
fn3, I made these the same code path.

This patch also introduces GlobalKey, though the GlobalKey logic isn't
hooked in yet.

This is Hello World in the new world:

```dart
import 'package:sky/src/fn3.dart';

void main() {
  runApp(new Text('Hello World!'));
}
```

- I extracted the BuildScheduler into a separate binding.dart file.
- Various changes to expose private members that are needed by
  binding.dart.
- Registering the render objects for event dispatch.
- Convert the tests to use the new binding mechanism.

This doesn't yet have a RenderView or event handling.

Add a LeafRenderObjectElement, fix a test to use the new contract.

- adds dartdocs
- replaces config setter with didUpdateConfig() so that you can't replace
the config outside of the system
- renames didUnmount() with destroy().
- rename Component to StatelessComponent, ComponentConfiguration to
StatefulComponent
- move debug dump to end of file
- renamed _holder to _element