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

This patch changes the framework to walk the child list forwards so that build
functions with global side effects do sensible things. Specifically, if you
have a number of autofocusable children, the first one the list will acquire
the focus because it gets built first now.

Fixes #182

RawEditableLine is now responsible for the scrolling behavior, which
removes the need for callbacks between RawEditableLine and Input. It
also fixes a bug whereby the whole Input widget (including its icon)
would scroll when the text got long.

This patch restructures how we size the editing region of the Input widget. Now
RenderEditableLine understands that it's a single-line editing widget and sizes
itself correctly.

Now you can colorize an Icon simply by proving a Color for the Icon.

There should be no more uses of Performance or AnimatedValue in the framework
or the examples.

RenderBlock wasn't constraining the results.
RenderPadding wasn't constraining the results (which matters
especially when the constraints can't fit the padding in the first
place).
RenderViewport wasn't constraining the results.

Add a test for the block case.

To catch this kind of thing in the future, add some asserts to
debugDoesMeetConstraints() that all four intrinsic functions return
values that are within the constraints.

RenderBlockViewport doesn't support returning intrinsics, so turn off
the "no intrinsic support" asserts (and return zero) when we're doing
this new assert.

This new assert screwed up the custom layout classes' tests, so adjust
those tests to ignore the callbacks invoked from these asserts.

Add to the _debugReportException() method a short summary of the
descendants of this node. It's important to have this information when
debugging errors like these intrinsic constraints contract violations
because often nodes just pass the values through to their child so you
have to go several steps down to find the actual problem.

Fixes https://github.com/flutter/flutter/issues/1210

Also, some trivial fixes for things that I found while playing with
the rendering library directly.

And rootConstraints to configuration.

This type and its variable have nothing to do with Constraints.

Fixes https://github.com/flutter/flutter/issues/829

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.

No you can't hit things inside degenerate transforms. Tranforming from global
coordaintes to degenerate local coordinates gives you Point.origin. Also,
upgrade vector_math to get a better invert constructor.

Fixes #1161
Fixes #1224

Also, update callers of deprecated methods.

We were painting the contents of the RRect clip at the wrong offset. This patch
makes RRect match Rect and Path clips.

Fixes #1194