Turns out many of the functions on BoxConstraints weren't used or had callers
that could easily be updated to other functions. I've added dartdoc to all the
public functions as well as renamed some functions that had similar names but
did different things.

Center and Align should expand by default

This patch makes Center and Align expand by default, which is usually what you
want. It also adds a ShrinkWrap option to let you shrink wrap in one or both
directions if that's really what you want to do.

Add Dismissable unit tests

Add coverage for the DismissDirection paramter.

These needed to be updated because we moved the widgets directory into the src
directory to hide it from dartdoc.

Handle Widget instances being moved as-is to different parts of the tree.

Setup mac target for Fitness app

Improvements to demo game

They're just as crazy in this system as they are in the web.

```
enum DismissDirection {
  vertical,
  horizontal,
  left,
  right,
  up,
  down
}
```

To only enable dismissing to the right create the `Dismissable` with `direction: DismissDirection.right`. By default direction is `DismissDirection.horizontal` (left or right).

Updated the card_collection "Swipe Away" demo with a drawer that can be used to select one of the three X axis dismiss directions. Currently the MixedViewport class doesn't support horizontal scrolling, so the demo doesn't support the X axis dismiss directions.

Demo game improvements

Events should bubble up the tree

Error out on start if nothing started.

(optional param default value + newlines)

Previously we reversed the event path by mistake before dispatching events.

widget is actively ignoring or not by changing a boolean instead of
the tree structure. ignoring defaults to true.

defaultHitTestChildren should return whether it hit something

Some clients want to know whether they hit their children.

Fix minor problems in _ScrollGestureRecognizer, Dismissable, lerpColor

Alternating scroll gestures would sometimes be ignored because _ScrollGestureRecognizer didn't always reset its _state when the pointer[s] went up.

A Dismissable dismiss triggered by a drag and then a fling could cause the next attempt to drag-dismiss to fail.

Fixed the definition of lerpColor().

Alternating scroll gestures would sometimes be ignored because _ScrollGestureRecognizer didn't always reset its _state when the pointer[s] went up.

A Dismissable dismiss triggered by a drag and then a fling could cause the next attempt to drag-dimiss to fail.

Fixed the definition of lerpColor().

Adds power bar and movements to boss fights in demo game

These functions are now in sky:dart.

RenderFlex should handle overconstrainted constraints

Disable reparent during sync

This feature is causing a bug because the widget tree isn't correctly
synchronized with the render tree.