This test causes the buildbot to hang. We don't currently have any hang
protection, which means this test ends up killing the bot. Disable the
test for now why we work on the root issues.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

These are now part of material.dart.

Fixes #1084

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

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

Also, rename build_utils.dart to widget_tester.dart. These files are now named
for their most commonly used classes.

Finally, add a .analysis_options to silence the (intentional) analyzer warnings
in append_child_test.dart.

Most of this work in this patch is building out the test framework to the point
where we can write this test.