anchor.diff:  const isException = const _Exception();
   
   /** A matcher for functions that throw Exception */
  -const Matcher throwsException = const _Throws(isException);
  -
  -class _Exception extends _ExceptionMatcher {
  +const Matcher throwsException = const Throws(isException);
  +
  +class _Exception extends ExceptionMatcher {
     const _Exception() : super("Exception");
     bool matches(item, MatchState matchState) => item is Exception;
   }
  
positive.code:  * object matches [matcher]. If [matcher] is not an instance of [Matcher], it
   * will implicitly be treated as `equals(matcher)`.
   */
  Matcher throwsA(matcher) => new _Throws(wrapMatcher(matcher));
  
  /**
   * A matcher that matches a function call against no exception.
  
negative.code: 
  /** A matcher for functions that throw Exception */
  const Matcher throwsNotImplementedException =
      const Throws(isNotImplementedException);
  
  class _NotImplementedException extends ExceptionMatcher {
    const _NotImplementedException() : super("NotImplementedException");
    bool matches(item, MatchState matchState) => item is NotImplementedException;
  }
  