Be more generous with our sourcemap discovery

The sourcemap indicator line doesn't have to be the last line in the
document, so it's not too safe to assume so.

jQuery, for example, puts it at the top.

Spec: https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.txk3vdsh99hf