Was trying to be consistent with later in the file, the first statement in the switch statement is an inverted if statement. See later in the file:
 * https://github.com/mithro/web-animations-js/blob/7d02b0f20e9652836cbb0b918d1055416c6f324d/test/test-runner.html#L369 
 * https://github.com/mithro/web-animations-js/blob/7d02b0f20e9652836cbb0b918d1055416c6f324d/test/test-runner.html#L407