Re: [dom] matches, querySelector, etc. shouldn't throw on an unrecognized selector (#39)

1. I think we're avoiding a `Selector` object because implementations intern the string version anyway, so there's not really anything meaningful to be gained.
2. Is null better than the empty string? Also, we could update `matches()`, `qS()`, and `qSA()`, but I doubt the user agent will know when they're passed a "clean selector" so you'll still have the perf-cost from them potentially throwing. But it might be nice for authors to recognize null/the empty string there. 
3. `q()` and `qA()` can of course do the right thing either way with support for a "clean selector".

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