* MultiTapGestureRecognizer previously would assert if there was no
competition.
* GestureArenaTeam would always select the first recongizer as the
winner even if a later recognizer actually accepted the pointer
sequence.
* debugPrintStack would fail a type check if maxFrames was non-null.
* FractionalOffset.lerp would throw a null-pointer exception if its
second argument was null.
Also, add a number of tests for previously untested lines of code.
Also, make hasOneLineDescription more discerning.
Also, add a test for hasOneLineDescription.
Also, add a test for GravitySimulation, to test the toString.
