This introduces the concept of an Anchor, which you can use to link
transitions together. I've used this in the Fitness and Stocks apps to
link the FAB and SnackBar to animate together by sharing the
SlideTransition.
I also fixed the Scaffold hit testing code to apply sub-widget
transforms, so it works with Transformed nodes.
Now it wont crash on save, but it still doesn't display
the error message in a snackbar and this also doesn't
wire up the enter/go key to actually do anything.
We'll need to implement performEditorAction on our
InputConnection implementation to catch this key and
pass it along to dart.
Our Input control needs to get more powerful to handle things
like this, including filtering of input.
The code was crashing by triggering a bug in the SnackBar
which didn't assert for actions, but crashed when they
were not null.
Partial fix for https://github.com/domokit/sky_engine/issues/543
@abarth
