function SourceMapGenerator(aArgs) {
    this._file = util.getArg(aArgs, 'file');
    this._sourceRoot = util.getArg(aArgs, 'sourceRoot', null);
    this._sources = new ArraySet();
    this._names = new ArraySet();
    this._mappings = [];
    this._sourcesContents = null;
  }
An instance of the SourceMapGenerator represents a source map which is
being built incrementally. To create a new one, you must pass an object
with the following properties:

  - file: The filename of the generated source.
  - sourceRoot: An optional root for all URLs in this source map.