anchor.diff:  class _Throws extends BaseMatcher {
     final Matcher _matcher;
   
  -  const _Throws([Matcher matcher = null]) : this._matcher = matcher;
  +  const _Throws([Matcher matcher]) :
  +    this._matcher = matcher;
   
  -  bool matches(item) {
  +  bool matches(item, MatchState matchState) {
       if (item is Future) {
         // Queue up an asynchronous expectation that validates when the future
         // completes.
  
positive.code:     try {
        item();
        return false;
      } catch (final e) {
        return _matcher == null || _matcher.matches(e);
      }
    }
  
  
negative.code: 
  class _InvalidOperationException extends _ExceptionMatcher {
    const _InvalidOperationException() : super("InvalidOperationException");
    bool matches(item, MatchState matchState) => item is InvalidOperationException;
  }
  
  class _NullArgumentException extends _ExceptionMatcher {
    const _NullArgumentException() : super("NullArgumentException");
    bool matches(item, MatchState matchState) => item is NullArgumentException;
  }
  
  //
  