* @throws IllegalArgumentException if any of the parameters were invalid
   */
  public PercentEscaper(String safeChars, boolean plusForSpace) {
    // TODO(dbeaumont): Switch to static factory methods for creation now that class is final.
    // TODO(dbeaumont): Support escapers where alphanumeric chars are not safe.
    checkNotNull(safeChars); // eager for GWT.
    // Avoid any misunderstandings about the behavior of this escaper