anchor.diff:          bool hasMore = formalBuilders.moveNext();
           assert(hasMore);
           VariableDeclaration realParameter = formalBuilders.current.target;
  -        Expression initializer = parameter.initializer ?? new NullLiteral();
  +        Expression initializer =
  +            parameter.initializer ?? astFactory.nullLiteral(null);
           realParameter.initializer = initializer..parent = realParameter;
         }
       }
  
positive.code:           library.uri.path == "_builtin" &&
            member?.name == "_getMainClosure") {
          // TODO(ahe): https://github.com/dart-lang/sdk/issues/28989
          return new NullLiteral()..fileOffset = offsetForToken(token);
        } else {
          return new UnresolvedAccessor(this, n, token);
        }
  
negative.code:   @override
    void handleLiteralNull(Token token) {
      debugEvent("LiteralNull");
      push(astFactory.nullLiteral(token));
    }
  
    @override
  