remove source root for filename

Currently, this implementation is causing the path to double up - `some/file/url.js` becomes `some/file/url.js/some/file/url.js`.

The source root is optional, and only necessary if there are a number of source files that would benefit from having a common root.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1226

Closes #1226.