First pass on sprite physics node hierarchy integration

Fixes #1702

Adds EnterExitTransition widget

This widget smoothly resizes as its child changes and lets you provide entrance
and exit transitions for its children.

Close the UrlLoaderProxy on error.

RenderOffStage test

Also, give RenderViewport a default offset.

Fixes #1680.

I accidentally lost this when extracting RenderOverflowBox from
RenderProxyBox.

Introduces a new Widget that lays its child out, but does not render it.
Also, moves RenderOverflowBox to its own file since it has nothing to do
with proxy box really.

We call this function _getInnerConstraints in other classes. Also, this change
makes it private for RenderOverflowBox.

The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

Provide a way to schedule a callback for immediately after the current
frame has been sent to the GPU thread. This is useful for times where
you want to immediately schedule yourself for another build or layout
because, for instance, you just displayed frame zero of an animation and
you want to use the metrics from that layout to set up the actual
animation to begin immediately, such that the very next frame is frame 1.

Fix analyzer warnings in examples

Add support for verifying .flx signatures when updating

Adds a step to the updater to verify that the new .flx package is signed and
untampered.

Each .flx contains a signed manifest file. The manifest contains a SHA-256 hash
of the .flx contents. See bundle.dart for a description of the new .flx format.

Show ColorFilter

Instead of requiring clients to use ui.ColorFilter, we show show the
ColorFilter class, similar to what we do for Color.

Renames PhysicsNode to PhysicsWorld

Convert Align to use FractionalOffset instead of two doubles

It turns out that an AnimatedContainer with a BoxDecoration would
start animating every single time it was built, whether or not the
container's decoration had changed.

It was confusing as to whether these were fractions or pixels.

Fixes #1622

Remove ColorFilter widget

This widget is slow and not commonly used. The original reason we added it was
to colorize images, but now that feature is built into the image and background
image widgets.

Fixes #1646

Makes sprite physics bodies scalable

Remove old setFoo functions on Paint

We now just use Dart properties, which is more idiomatic.

Fixes #1669

They should be 0.0 when there's no offset.

Fixes #1661