1 This tests that custom properties on element.dataset persist GC.
3 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
6 PASS d.getAttribute('data-custom-property') is '1'
7 PASS d.dataset.customProperty is '1'
8 PASS d.dataset.customProperty is '1'
9 PASS successfullyParsed is true