Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

Previously, hitTestChildren in RenderBlockViewport just didn't support
horizontal scrolling, due to an oversight.

Makes PageableList with itemsWrap:true work again.

Plumbed the itemsWrap parameter through to HomogenousViewport.

Fixes issue #877.

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

We now have separate gestures for horizontal, vertical, and pan scrolling.

Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class.
Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.

These gestures use the new gesture detection system.

Also includes changes to widget_tester sufficient to
test scrolling.

@abarth