It duplicates a comment

I discovered this when using flowts, which runs `babel-plugin-flow-to-typescript` through recast.
https://github.com/zxbodya/flowts/issues/10

The babel plugin splits up this import statement into a regular import and a `type` import, and deletes the comment.

Recast retains the comment, and puts it before both lines.

This isn't terrible, but it's interesting.

Start by reproducing the linked flowts case involving babel-plugin-flow-to-typescript, recast, and a commented import statement. Trace recast's comment handling around the split import and compare the output with the input; done means the transformation no longer produces the reported duplicated or misplaced comment.

Written by the indexing model from the issue text.