- 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.

- move _uniqueChild earlier since a comment now mentions it earlier.
- reorder methods in Element to more closely reflect call order.
- change mount to be the place that sets the parent pointer, for consistency.
- make the lifecycleState a purely debug-time thing for consistency.
- make BuildableElement.unmount set dirty to false so that we won't
  build unmounted objects.
- rename "updated" to "newWidget" for clarity.
- change how we unmount things so that the slot is reset up to a
  RenderObjectElement, but not further, and don't reset the depth.

All the use cases for EventDisposition are now addressed by the gesture
detection system.

We no longer have gesture events.

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

fn3 shouldn't rebuild components that don't change identity

add a test for scale gestures

* If no source path is provided, then run the analyzer on the Sky unit tests
* Fix the filter for errors found in pub cache packages
* Generalize the filter for the analyzer's "xx errors/warnings/hints found" status message

Also fix a test that caused a warning in the analyzer.

Previously EditableText would render a text widget with no cursor if the text
value was empty.

Also adjust the height of the cursor widget to reflect the style's line
height, and update the cursor painting to match.

Previously --stage-two would error out because the LICENSE was git ignored.
This patch includes the LICENSE file in the repo to avoid the error. Also,
refactor the big_red_button.py to be clearer about what's shared between stage
one and stage two of the release process.

Prototype of fn3

We were just missing a few type casts in tests to make the analyzer happy.