mozilla/source-map — pull request #104 by lydell (2014-03-14 19:33:50+00:00)
Title: Don’t add `file` property if no file was given

Previously, if you didn’t pass the `file` option to
`new SourceMapGenerator()` or
`SourceNode.prototype.toStringWithSourceMap()`, `"file": null` would be
present in the source map. Now, the file property isn’t added at all
instead, which is more intuitive.