Add support for saving settings in fitness app

The compositor backends we're planning to use can't handle a general-purpose
paint layer and instead need lower-level operations.

Fixes #707

Key should be const

For great constness.

Fixes #693

We now use the term `renderObject`.

Fixes #708

- Introduce some new Layer classes.
- Introduce paintChildWith* methods.
- Convert paint() methods to use paintChildWith* where appropriate.
- Fix paintBounds logic in Layer world.
- Introduce Layer.replaceWith(), so that it's clearer what's going on.
- Make RenderObjects have a ContainerLayer, not a PictureLayer.
- Introduce a PaintingContext.replacingLayer() constructor to highlight
  where we are creating a layer just to replace an older one.
- Rename some layer-related methods and fields for clarity:
   requiresCompositing -> hasLayer
   hasCompositedDescendant -> needsCompositing
   updateCompositing -> updateCompositingBits
   _needsCompositingUpdate -> _needsCompositingBitsUpdate
   markNeedsCompositingUpdate -> markNeedsCompositingBitsUpdate
- After updating compositing bits, if we find that the bit changed, we
  now call markNeedsPaint().
- Reorder markNeedsPaint() logic for clarity.
- Make flushPaint() start at the deepest node.
- Make _compositeChild() avoid repainting children with hasLayer that
  aren't dirty, instead it just reuses their existing layer.
- Made RenderView reuse the RenderObject layer instead of having its own.
- Made RenderView have hasLayer set to true.
- Add various asserts and comments.

Changed the pageable_list.dart example: tapping on the toolbar changes
the scroll direction. This exposed some problems:

- Scrollable.syncFields() didn't update scrollDirection
- Viewport updated its RenderObject fields in the wrong order
- FixedHeightScrollable scrollDirection changes didn't update the scrollBehavior

There may be similar problems with VariableHeightList and ScrollableViewport.
I will fix those in a separate CL.

Rather than using a Padding widget to provide padding along the scrolling axis,
we now just figure the padding into where we draw the items. This patch fixes
an issue where we would remove the first topmost item in a scrollable list too
early because we thought it was already off screen.

Fixes #697

- Clarify comment about RenderObjectWrapper.
- Assert that we never sync a RenderObjectWrapper with an instance of a different type.
- Assert that MultiChildRenderObjectWrapper subclasses always have multi-child RenderObjects.
- Assert that renderObject doesn't change identity when syncing.
- Make searchForOldNode() return void since the return value is ignored.
- Remove code that handled renderObject changing during sync.

Dismissable: correct handling of a drag that ends with a fling

Give default scroll directions

Fixes the Stocks crash on startup.

Fixes tests.

Eventually we'll plumb positionX and positionY out so that developers can
control them.

Fixes the tests because previously we wouldn't draw any box decorations.

Add the ability to draw a foreground box decoration

- 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 feature lets you create effects similar to the "outline" property in CSS.

Pick a default theme brightness

... and reattach fitness to the build. We detached fitness from the build
because it was seeing an old version of playfair that didn't work with the most
recent version of the Sky package.

There doesn't seem to be any particular reason for us to not default the brightness to _something_.

Unhook fitness from the build to work around build break in playfair. We
updated the Sky package in an incompatible way, which means we need to roll in
a new playfair package to build fitness.

Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.

Split box.dart into many files

Sadly, box.dart has grown much longer than 1000 lines. This patch splits it up
into several files based on the class hierarchy. Fortunately, many of these
classes are loosely coupled to each other.

When given a null image, RenderImage should be as small as possible (isntead of
being NaNxNaN).

Also, plumb ImageFit and ImageRepeat through the various image widgets.

This CL inflates the padding of Container to account for the borders so that
the borders are allocated space in the layout and don't draw behind the
Container's child.

Request gridlines for fitness chart

We now fold the width and height properties into the constraints and try to
find a size for the image that perserves the image's intrinsict aspect ratio
while fitting within the given constraints.