Fix ReactEmptyComponent disappearing and throwing in IE8

One possible fix for https://github.com/facebook/react/issues/1494 and at the same time enabling the safe use of non-visible tags in IE8 for React components as well.

The down-side is cost, http://jsperf.com/testnoscrip, in worst-case scenarios it's 20% slower, that is; there's no leading text and only one element being inserted. However, using a more precise test (with no hits) the overall performance seems to only dip by 6%. Looking at http://jsperf.com/testnoscrip/3 with 6 elements being inserted, the cost of the lookups becomes reduced to only 2% slower overall performance.

My PR uses the old slower fix for now, pending your input on this issue.