cycles make invalid imports work

[bmeck]

// a.mjs
import './b.mjs';

// b.mjs
import {DOES_NOT_EXIST} from './a';

[jdalton]

The fix d38f0db is not super great, but it does validate the entry once it finishes loading. It could benefit from #46. I'll be adding a round of unit tests over the next couple of days to solidify support.