Adds a HomogeneousViewport class that works like MixedViewport but
handles only children that have all the same height.

Converts ScrollableWidgetList to use that, so that we don't waste a
frame looking at the size of the contents each time we change size.

This allows a number of seemingly pointless double-pumps in the tests
to be removed.

Other changes that were necessary to support the above:

 - RenderBlock now supports minExtent (think 'min-height' in CSS)
 - RenderBlock now supports itemExtent (forces the height of each
   child to be the same, so that the itemExtent passed to the fixed-
   height scrollables are all authoritative instead of a source of
   bugs when they don't match)
 - RenderBlockViewport now supports horizontal scrolling
 - improved the style of the isInfinite assert in box.dart
 - fixed the position of a comment in mixed_viewport.dart
 - added a test
 - made the logic for how many items to show be more precise