code: function normalizeFunctionCall(tokens, start) {
      var parenDepth = 1, end = start + 1, n = tokens.length;
      while (end < n && parenDepth) {
        // TODO: Can URLs appear in functions?
        var token = tokens[end++];
        parenDepth += (token === '(' ? 1 : token === ')' ? -1 : 0);
      }
      return end;
    }
docs: Normalize tokens within a function call they can match against
  cssSchema[propName].cssExtra.
  @return the exclusive end in tokens of the function call.