If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)
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.
