This patch makes it easier to use the auto layout API:
* We no longer use operator== because that requires an ugly cast by the
API user.
* Also, "leftEdge" is now just "left" for less verbosity.
* AutoLayoutChild not implies its key from the AutoLayoutParam object.
* We now correctly layout every child of a RenderAutoLayout object even
if the solver doesn't flush any updates to that child.
This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.
Previously, hello_world.dart was an interactive circle. I've moved that
to touch_input.dart. We should eventually harmonize the touch input
examples at all the layers.
