Add listen command to sky_tool, and related changes.

Make sky_tool install to ios devices and simulators if available, and add a ‘listen’ command that will listen to filesystem changes in the current directory and update running versions of the app.

Also refactors ios_sim.py to be part of sky_tool.

The current skyx file that ‘listen’ generates does not contain material design icons, so icons will be missing.

This patch adds a new test harness and a first, trivial test to run with the
harness. The new test harness is built on package:test and should run on
Travis. Over time, we'll migrate our existing tests into this harness.

This also changes the way we insert nodes into a
MultiChildRenderObjectWrapper's renderObject, which fixes issue #626.
Now, instead of the slot being a renderObject, it's the Widget that
currently uses that renderObject. That way when the Widget changes
which renderObject to use, the siblings of that Widget in the same
child list don't have to be notified of the change.

I tested performance of the new algorithm vs the old algorithm using
the stocks demo at idle and the stocks demo scrolling steadily. The
data suggests the algorithms are roughly equivalent in performance.

Rationalizes this name with walkChildren.

This function just calls remove(). Also, have Widget do the recursive remove
walk by calling walkChildren.

Add goal weight indicator line to fitness chart

Workaround for Drawer.initState() fails

This is a hack to avoid #715

In theory, the navigator's state should be updated in the event handler that triggers opening the drawer. It's not obvious how to do this, since we only build the Drawer when it's supposed to be visible (and once we're building it's too late).

Support for saving goal weight

Stack squishes positioned elements near the edge

Remove RenderView.paintFrame

We now use the repaint system to do all the painting. During initialization, we
set up a root layer that applies the device pixel ratio.

Fixes #706

When laying out positioned children inside a stack, we should give them
unbounded constraints because if they draw outside of the stack, we'll just
clip them.

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.