code: function sourceRootFn (file) {
    let sourcePath    = file.history[0],
        targetPath    = path.join(__dirname, '../src/'),
        relativePath  = path.join(path.relative(sourcePath, targetPath), './src');
    return relativePath;
  }
docs: Composes source-maps' sourceRoot property for a file
  @param {any} file File being processed
  @returns {any} sourceRoot value