Fix asserts related to image drawing.

There were two problems:

1) When starting the mimic, we put up a bad frame because although we set the
   `begin` value of the animation, we were building using the current `value`,
   which hadn't been updated.

2) When stoping the mimic, we'd dirty a component during didUnmount, which
   wouldn't get cleaned until the next frame. Now we're sure to clean all the
   components before leaving flushBuild.

Also, make RenderStack support negative positions with clipping so that we can
expand objects that are partially offscreen.

First example pubspec

Specify local build paths when using sky_tool.

directory to support using sky_tool with local
builds.

Add a --checked argument to sky_tool start

The --checked argument runs SkyShell in checked mode, which turns on type
checking and asserts.

Use the new virtual directory flag for sky_server instead of a symlink to prevent infinite subdirectory recursion

Adds perspective projection to Node3D in sprites

This widget lets you mimic one of its children in an overlay. The overlay
starts out as the same size of the child and then grows to fill the overlay. In
the future, the mimic will start at the same visual position as the child.

Unbreak text

Previously, no text was appearing because we weren't actually converting the
Dart objects into DOM.

Adds skew property to sprite node

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