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!'));
}
```

Add TagWidget to fn3

Currently, fling scrolls animate far too long after the display has effectively stopped moving. Use the ScrollSimulation's tolerance property to stop the scrolling roughly when the display stops changing.

Fixes errors from #1285 and #1281.

- 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.

We'll need tag nodes before finishing Flex and Stack.

Fix scrolling of Block.

Since our build function depends on scrollBehavior.isScrollable, any
time we update scrollBehavior we are implicitly updating our state. As
such, we must do so during a setState() call, or else we won't rebuild
and might not bother to listen to the scroll gestures.

This probably broke when we made Block not listen to gestures if it
wasn't overflowing.

There are some TODOs in this file that I'll address in the next patch.

More dartdocs for fn3, refactor buildDirtyElements

Ensure that EditableText always shows a cursor

Some cleanup of the Sky analyzer script

The buildDirtyElements() method now enforces that we only build deeper
nodes and never go back to build higher nodes during a frame.