I'm not sure this is the correct approach, but it
at least mitigates https://github.com/domokit/sky_engine/issues/804
for now.

I also made it not crash if you haven't set a goalWeight.

@collinjackson

Mostly formating and removing arguments with default values.

These widgets were designed in CSS where you need to specify a layout model for
your children. This patch updates them to the modern style of just taking a
unique child.

Fixes #755

Block -> BlockBody
ScrollableBlock -> Block
FixedHeightScrollable -> ScrollableWidgetList
VariableHeightScrollable -> ScrollableMixedWidgetList
BlockViewport -> MixedViewport

Fixes #743

- PageableList extends ScrollableList
One fixed width or height item is visible and centered at a
time. Fling and drag gestures scroll to the next/previous item.

- Scrollable.scrollTo(), Scrollable.scrollBy(), ensureWidgetIsVisible() API changed
The named animation parameter for these methods was replaced by
duration and curve. All of the methods now return a Future. The Future
completes when the scroll does.

This change eliminates the need for Scrollable to temporarily take ownership
of a ValueAnimation object (see #645).

- Using Future.then() instead of an AnimationPerformance status listener
In ensure_visible.dart _handleTap() uses ensureWidgetIsVisible() to
center the card roughly as before and then. When the implicit scroll
animation is complete, it changes the centered card's label font. The
change is made when the Future returned by ensureWidgetIsVisible()
completes.

- FixedHeightScrollable's itemHeight parameter is now itemExtent
If scrollDirection is ScrollDirection.vertical (the default) then itemExtent should
be the height of each item; otherwise it should be the width of each item.

Replaced _velocityForFlingGesture() in scrollable.dart with Scrollable._eventVelocity()
The original version clamped pixels/ms against pixels/sec constants. The new version
also deals with scrollDirection.

- Plumbed scrollDirection though FixedHeightScrollable and ScrollableList

Both classes should now support horizontal scrolling.

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.

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

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

Fixes https://github.com/domokit/sky_engine/issues/547

@collinjackson

Fixes https://github.com/domokit/sky_engine/issues/548

@collinjackson

Fixes https://github.com/domokit/sky_engine/issues/540

@abarth

This required me wrapping the new Activity APIs
as well as adding the missing pubspec.yaml

@abarth