This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.

...so that it's easier to copy/paste between the rendering and settings layers.

I meant to check this in as part of https://github.com/flutter/flutter/pull/1852
but forgot to commit the local change, d'oh.

This makes it match the material spec more.
https://www.google.com/design/spec/style/icons.html

Fixes https://github.com/flutter/flutter/issues/1357

This library is being folded into dart:ui.

Fixes #83

Previously we had totally wrong behavior. Now we have more correct behavior and
testing.

Fixes #1808

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

The asserting will continue until morale improves!

* Convert all assert(*.isNormalized) checks to use the new
  debugAssertIsNormalized technology.
* Convert CustomMultiChildLayout to use the new RenderingError
  technology to greatly improve the detail in errors you get when
  writing CustomMultiChildLayout delegates.
* Add BoxConstraints.copyWith().
* Indent the descendants in the rendering exception data dump so that
  when you have multiple children it's clearer what's going on.

Negative constraints never make sense, so catch those too.

Make RenderObject.layout's isNormalized assert use the newer more fancy
debug version of isNormalized.

Factor out the HitTestBehavior logic so that RenderMetaData can use it.

Use that in DragTarget.

This patch teaches block how to anchor its scrolling to the end rather than the
start.

Fixes #136

These classes now share more code and have feature parity.

The "leaf merge" feature was getting confused when we reset the
SemanticsNode. We now separately track whether the node itself is marked
as being merged vs whether we inherited that state, and we don't reset
the inherited state until you're reattached or reserialised. In the
latter case, we do a "just in time" clearing of the flag just like we
previously did a "just in time" setting of the flag, except now the flag
we're setting or clearing is the inherited flag not the actual flag.

There aren't any clients anymore.

Fixes #1684

Now that the engine lets us take the view host handle, we can actually
access the view host from Dart.

This patch includes documentation for transitions.dart and for
scrollable.dart.

The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

* Use actual exceptions rather than assertions containing code
  containing strings when trying to give messages to authors.
* Introduce RenderingError which is an AssertionError that takes a
  string argument, to support the above.
* Provide a BoxDimensions.hasBoundedWidth/hasBoundedHeight API.
* Document BoxDimensions.isNormalized.
* Provide more useful information when we assert isNormalized and find
  that it is false.
* When finding the size is infinite, crawl the tree to figure out which
  render box is likely responsible for the infinite constraints.
* Provide more information when size doesn't match the constraints.
* Provide more information when intrinsic dimension methods violate the
  constraints.
* Only spam a huge amount of information for the first exception from
  the rendering library. I've noticed a lot of people looking at the
  last exception printed rather than the first and that's very
  misleading -- after the rendering library hits an exception, all bets
  are off regarding what'll happen in the future. All kinds of asserts
  might fire.
* Improve docs around the debug methods and flags for the above.
* Make Block default to have no children. Previously, giving no children
  crashed with a confusing message about a null deref in an assert.

This patch sketches out the basic widgets for creating Mozart child
views. We're still missing some of the machinery, so we can't quite
create child views yet, but once we supply the ViewHost and teach the
compositor how to actually display the child scenes, we'll have
something that works.

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

We were getting confused about our layer tree offsets, which caused us
to set an incorrect cull rect for the recording inside a viewport.

This patch does away with layer tree offsets almost entirely. We now use
them only at repaint boundaries, which is where we want the layer tree
to be mutable.

Fixes #1460

Port the tests to use the new interface.

They were out-of-date.

Fixes #1391

Now the text input control knows how to move the caret when you tap
inside the string. There's still some rough edges to polish up, but this
patch is the first step.

Fixes #108

These were introduced by the recent engine roll.

This patch teaches the editing system to paint reasonable selections for
single-line text fields, including for bidirectional text.

Requires https://github.com/flutter/engine/pull/2318

This patch changes the default alignment of OverflowBox to center rather than
having a bias towards the upper-left corner.

Fixes #1236