anchor.diff:   * for each exception type.
    */
   
  -/* abstract */ class _ExceptionMatcher extends BaseMatcher {
  +/* abstract */ class ExceptionMatcher extends BaseMatcher {
     final String _name;
  -  const _ExceptionMatcher(this._name);
  +  const ExceptionMatcher(this._name);
     Description describe(Description description) =>
         description.add(_name);
   }
  
positive.code: 
  /** A matcher for functions that throw IllegalJSRegExpException */
  const Matcher throwsIllegalJSRegExpException =
      const _Throws(isIllegalJSRegExpException);
  
  class _IllegalJSRegExpException extends _ExceptionMatcher {
    const _IllegalJSRegExpException() : super("IllegalJSRegExpException");
    bool matches(item, MatchState matchState) => item is IllegalJSRegExpException;
  }
  
negative.code:  */
  const Matcher returnsNormally = const _ReturnsNormally();
  
  class Throws extends BaseMatcher {
    final Matcher _matcher;
  
    const Throws([Matcher matcher]) :
      this._matcher = matcher;
  
    bool matches(item, MatchState matchState) {
  