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

Fixes #1808

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.

These classes now share more code and have feature parity.

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

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 use the ScrollDirection for more than just scrolling.

Fixes #151

This patch moves PageableList off HomogeneousViewport and onto RenderList and
friends, making it match the new ScrollableList.

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

This patch implements the remaining missing features of ScrollableList2.
It should now be nearly a drop-in replacement for ScrollableList. The
next patch will switch callers over to the new machinery.

ScrollableList2 uses the same pattern as ScrollableGrid, which requires the
client to allocate widgets for every list item but doesn't inflate them unless
they're actually needed for the view. It improves on the original
ScrollableList by not requiring a rebuild of the whole visible portion of the
list when scrolling. In fact, small scrolls can often be handled entirely by
repainting.