code: private function fixAssertNegative(Tokens $tokens, $index, $method)
      {
          static $map = [
              'false' => 'assertNotFalse',
              'null' => 'assertNotNull',
              'true' => 'assertNotTrue',
          ];
  
          return $this->fixAssert($map, $tokens, $index, $method);
      }
docs: @param Tokens $tokens
  @param int    $index
  @param string $method
  
  @return null|int