Add support for charts to fitness app

Adds ensureWidgetIsVisible() function to scrollable.dart

Maximum bounds for the mimic in the overlay can now be specified.

Adds API docs to particle systems and removes redundant property

Set the scrollOffset of a widget's Scrollable ancestor so that the
widget is centered within the scrollable. A future CL will add
support for specifying exactly where the widget appears. The scroll
can be animated by specifying the animation: parameter.

Changed the duration Scrollable.scrollTo() parameter from a Duration
to an AnimationPerformance so that one can configure all aspects of
the animation. The caller may also listen to the animation to schedule
other work while it updates or when its status changes.
complete

This is just fallout from not having
https://github.com/domokit/sky_engine/issues/565

@abarth

Revert "Roll Dart forward"

@abarth

@abarth

Attempt to make our splashes closer to other Android apps

And also Material Design of course.

@abarth

Don't paint twice

In refactoring the way we generate the SkPicture, I accidentially made us
generate the SkPicture twice. This CL causes us to generate it only once.

Adds API docs to sprite Node and Node3D

Scaffold: animate the FloatingActionButton with the SnackBar.

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.

Adds API docs to sprite ColorSequence and Layer

Make Fitness tiles nicer

Adds documentation for sprite actions

Moved from Cards to Tiles and made the printing
of the dates nicer by using some code from a Dart SDK example:
https://github.com/dart-lang/sdk/blob/master/samples-dev/swarm/swarm_ui_lib/util/DateUtils.dart

I also built a UserData class to help keep saving/sorting
consistent as well as fixed the sort order to have most
recent at the top.

@abarth

Update README.md

Roll Dart forward

Fix crash when hitting "enter" key on keyboard during entry

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

Added missing :

Adding instructions for running examples.