RenderViewport should use integer device pixels

Rationalize text

Now they give you a hint as to what went wrong. Also, in checked mode, they now
throw during construction instead of during building.

Fixes #418

 - Inline -> Pargraph. This class is actually a box, not an inline. It's really
   a wrapper for RenderParagraph, so Paragraph is the normal name.

 - InlineBase -> RenderInline. The name we used in C++ for the base class of
   all inlines was RenderInline, which removes the ugly "Base" suffix.

 - InlineText -> RenderText. Aligns this name with C++.

 - InlineStyle -> RenderStyled. Matches the foregoing pattern.

When painting a scroll offset, RenderViewport should use integer device pixels
to prevent "shimmering" effects on images.

Fixes #486

Improvements to game performance tests

Unforutnately since Input won't show text if it doesn't start
with a letter, this actually makes the experiance slightly
worse, but this is definitely the right direction.

@abarth

Add widgets for reparenting widgets

Wrap widgets you want to reparent in a Mimicable widget and assign the
Mimicable widget a global key. Then, given the same global key to a Mimic
widget to make it appear elsewhere in the view hierarchy.

Roll mojo to https://github.com/domokit/mojo/commit/35de44e3618d508b2b3862af2216aee8c70ad8e5 

@abarth

They settle much faster after you fling them now.

We only use them for the transition to dismissed.

Push PopupMenu's animation code into its build function and use trans…

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

... and use them to improve the horizontal_scrolling example.

Now ScrollableBlock can combine a horizontally scrolling viewport with a
horizontal block.

Also rename ViewportScrollDirection to just ScrollDirection for less verbosity.

Teach Block how to layout horizontally

We'll need this for horizontally scrolling lists.

Add Fitness.apk with icon

This is just one of many steps in preparation for turning
fitness into a play-store ready app.

@abarth @collinjackson

Add a few bits of documentation to package:sky

restart Sky apps on Android without having to restart SkyShell.

Add a basic radial reaction to the switch

We still need to tune the various parameters, but this patch is a start.

workaround for Fitness demo dialog cancel bug

Adds cull rect to sprite performance test

I can't really make further progress until at least:
https://github.com/dart-lang/dartdoc/issues/766
is resolved, ideally also:
https://github.com/dart-lang/dartdoc/issues/763

@abarth

Initial version of game performance tests