Reimplement 'stretch' for flexible items correctly.

Cleanup how we manage scrollOffset in Scrollable

 - Introduce _setScrollOffset as a backend for the animations so that scrollTo
   can stop animations.

 - Create a single function that stops both kinds of scroll animations.

 - Refactor how we update the bounds for bounded scroll behaviors so that we
   update the bounds and compute the new scroll offset at the same time.

Fixes #698 to actually work.
Also, adds some debugging aids around Flex.
And a test to check this fix.

Don’t flash a message while fitness data is loading

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

Fixes #743

We now support uniform borders on circular box decorations.

Fixes #741

Migrate sky/tests/layout to sky/unit/test

Also, I've organized the tests by the render object they're testing.

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