Say you have a file called `src/thing.js` and you run
$ babel src/thing.js --out-file lib/thing.js --source-maps true
This generates a source map at `lib/thing.js.map` that contains
"src/thing.js" in its `sources` array. This is incorrect; since browsers
resolve all relative URLs relative to the directory containing the file
that refers to the URL, this resolves to `lib/src/thing.js`.
To make the source map refer to the source files correctly, the
`sources` array should contain "../src/thing.js".
