This changes how PopupMenu works slightly because there's no long an onPressed
callback on PopupMenuItem. Instead, callers should use showPopupMenu.

Updates for change in Mojom dart enum generation

Port most of scrollable.dart to fn3

Port some drawer and button widgets to fn3

Update to Mojo 4e4d51ce28a8edcb32b9c7f555e38e2ae84a825e, update deps

Fix the minedigger game.

This updates to mojo 4e4d51ce28a and mojo sdk 711a0bcfb141b4 and updates the sky
package's pubspec.yaml dependency to '>=0.1.0 <0.2.0' to be compatible with
the current mojo package. This includes an update to the Mojo Dart generator to
produce real classes for enums and the corresponding updates for users of the
KeyboardType enum in Sky as well as one scoped_ptr->std::unique_ptr in shell
corresponding to a change in the Mojo EDK.

When a new version of the sky and sky_services package are pushed this will fix
domokit/mojo#440.

Remove the Widgets framework's PointerEventListener definition now that
the Rendering layer has one. It was previously clashing in files that
imported both, which would result in it being hidden. Turns out
MineDigger is the only example that this affects.

This change ensures that we only store the results of a hit
test on the initial pointer down event.  Moreover, we perform
new hit tests each time a hovering pointer moves.  This is
important to ensure correct behavior of input devices which can
hover, such as mice.

Previously the first hover movement after releasing a mouse
button would cause a new pointer state to be recorded along with
hit test results for wherever the pointer happened to be which
caused the following pointer down event to be delivered to the
wrong place.

Fixes issue #1189.

Port DragTarget to fn3

Adds documentation and cleans up code

Fix sky_tool start/listen on iOS

Port some widgets to fn3

Replace C++ UpdateTask with an empty dart version, loaded as a snapshot

In this new world, Listener is just a wrapper around a node in the
render tree that hooks directly into the event handling logic.

This allows people to use localToGlobal and company.

This adds a new placeholder dart package updater in sky/packages. This is built
into a snapshot and compiled into the Sky engine binary using the same
mechanism as the Dart isolate.

I also added a SkyHeadless class, similar to SkyView, used for running Dart
code without a view.

Removes artifacts from textured lines in sprites

- Remove the unique objects used as slots since we decided 'null' was
  fine after all
- Rename 'slot' to 'newSlot' when it's used as an argument to change the
  _slot field, to clarify which variable has the newer value
- Remove the RenderObject registry since we'll do listeners a different
  way. This also removes handleEvent for the same reason.
- Remove the TODOs for mount/unmount becoming didMount/didUnmount since
  the methods do in fact do the mounting/unmounting.

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!'));
}
```