code: function parseUrl (ast) {
          let [firstAgument] = ast.arguments;
          let rawUrl = firstAgument.raw;
          let url = rawUrl.replace(/^\//, '').replace(/\/$/, '').replace(/\\/g,'');
          assert(url);
          return url;
      }
docs: /* The returned url should be the inner contents of the RegExp,
  with all escape sequences removed, so we strip out leading and
  trailing/characters, and any occurences of //.