anchor.diff:      return _matcher.describeMismatch(item, mismatchDescription, matchState, verbose);
     }
   }
  +
   
   class ThrowsMatcher extends Throws {
     final Matcher _matcher;
  
positive.code:   }
    
    Description describe(Description description) =>
        description.add('an instance of ${t.toString()}');
  }
  
negative.code: Matcher toBe(expected) => same(expected);
  Matcher instanceOf(Type t) => new IsInstanceOfTypeMatcher(t);
  
  Matcher toThrow(Type exceptionClass, String message) =>
    new ThrowsMatcher(new ComplexExceptionMatcher(instanceOf(exceptionClass), toEqual(message)));
  
  Matcher not(Matcher matcher) => new NegateMatcher(matcher);
  
  