anchor.diff:  Matcher not(Matcher matcher) => new NegateMatcher(matcher);
   
   
  -class NegateMatcher extends BaseMatcher {
  +class NegateMatcher extends Matcher {
     final Matcher _matcher;
   
     const NegateMatcher(Matcher matcher) : _matcher = matcher;
  
positive.code:   }
  }
  
  class IsInstanceOfTypeMatcher extends BaseMatcher {
    Type t;
  
    IsInstanceOfTypeMatcher(Type t) {
  
negative.code:   }
  }
  
  class ComplexExceptionMatcher extends Matcher {
    Matcher classMatcher;
    Matcher messageMatcher;
  
  