- Catch exceptions closer to the source.
- Factor out exception printing code.
- Have widget library hand the rendering library some context when syncing RenderObjectWrappers to aid with debugging.
- Fix a bug in flex.dart whereby _overflow was compared when null.

Fixes #698 to actually work.
Also, adds some debugging aids around Flex.
And a test to check this fix.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

This introduces the notion of event disposition and allows event
targets (widgets and render objects) to consume events that should not
be processed further. This is needed by the Switch component in the
Drawer in the stocks example. The Switch is embedded in a DrawerItem.
The Switch handles the gesture tap event to toggle its state and should
handle pointer events to allow swiping and draw its own radial
reaction. The DrawerItem also handles gesture taps to allow toggling
the switch value when tapping anywhere on the drawer and to draw its
own ink splash. When tapping on the switch, both the switch's render
object and the DrawerItem's listener are in the event dispatch path.
The Switch needs to signal in some fashion that it consumed the event
so the DrawerItem does not also try to toggle the switch's state.