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

RenderOffStage test

Also, give RenderViewport a default offset.

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

A MaterialList understands the sizing, padding, and scrollbar features of
Material Design lists.

Also, add CircleAvatar for showing the circular avatars that are commonly used
in material lists.

Reduce latency of low-volume debugPrint output

Add a AnimatedRelativeRectValue class for animating RelativeRects.

Add a PositionedTransition class for animating Positioned using
AnimatedRelativeRectValues.

Add a test for PositionedTransition.

Fix a math bug a RelativeRect found by the test.

Fix a logic bug in the two ParentDataWidget classes found by the test.
Specifically, they were marking the child dirty, rather than the parent.
The parentData is for the parent's layout, not the child's, so they have
to mark the parent dirty. (I didn't hoist this up to the superclass
because ParentData could be used for painting, hit testing,
accessibility, or any number of other things, and I didn't want to bake
in the assumption that it needed markNeedsLayout.)

- truncate pixel values to 1dp since there's really no point being told
  the Size is 302.98732587287 by 648.28498579187.

- describe more Widgets so that debugDumpApp() is more useful.

- remove bufferValue from ProgressIndicator (cc @hansmuller) since it's
  not yet implemented.

- half-hearted toString() for ThemeData. There's no point making a
  complete one, since it would cause line-wrap even on big monitors in
  debugDumpApp dumps, and you can easily get the actual values from a
  debugging if that's the issue.

- flesh out BoxConstraints.toString() so that fully unconstrained and
  fully infinite constraints are called out explicitly. I experimented
  with adding even more special cases, e.g. calling out unconstrained
  widths with fixed heights, etc, but it made the output less readable.

- remove a redundant _updateVariable() in AnimatedContainer (cc
  @abarth).

- add more information to RenderView.toString().

If we haven't printed anything for a second, then reset the count of how
much we've printed.

I also reduced the amount of data to ~16kb until we pause because I saw
some corruption even pausing 1 second every ~32kb. (Numbers are
approximate because we're counting UTF-16 characters and some of the
characters in the output are multiple bytes in UTF-8.)

Sundry debugging aids and fixes

Introduce a class that can describe the metrics used in a Positioned
widget, to enable easier manipulation and storage of the values, e.g.
for animation purposes.

(These are all the debugging-related fixes and trivial typo fixes that I
extracted out of my heroes branch.)

Fix rendering.dart import order.

Introduce a debugLabel for Performances so that when you create a
performance, you can tag it so that if later you print it out, you can
figure out which performance it is.

Allow the progress of a PerformanceView to be determined (but not set).

Allow subclasses of PerformanceView that are constants to be created by
defining a constant constructor for PerformanceView.

Introduce a debugPrint() method that throttles its output. This is a
test to see if it resolves the problems people have been having with
debugDumpRenderTree() et al having their output corrupted on Android. It
turns out (according to some things I read On The Internets) that
Android only has a 64KB kernel buffer for its logs and and if you output
to it too fast, it'll drop data on the floor. If this does in fact
reliably resolve this problem, we should probably move the fix over to
C++ land (where "print" is implemented) so that any use of print is
handled (avoiding the interleaving problem we have now if you use both
debugPrint() and print()).

Fix a bug with the debugging code for "size". In the specific case of a
RenderBox having a parent that doesn't set parentUsesSize, then later
the parent setting parentUsesSize but the child having its layout
short-circuited (e.g. because the constraints didn't change), we didn't
update the _DebugSize object to know that now it's ok that the size be
used by the parent, and we'd assert.

Also, allow a _DebugSize to be used to set the size of yourself.
Previously you could only set your size from a regular Size or from your
child's _DebugSize.

Add more debugging information to various Widgets where it might be
helpful.

Make GlobalKey's toString() include the runtimeType so that when
subclassing it the new class doesn't claim to be a GlobalKey instance.

Include the Widget's key in the Element's description since we don't
include it in the detailed description normally (it's in the name part).

Fix a test that was returning null from a route.