code: protected function includeMigrationFile($class)
      {
          $class = trim($class, '\\');
          if (strpos($class, '\\') === false) {
              if (is_array($this->migrationPath)) {
                  foreach ($this->migrationPath as $path) {
                      $file = $path . DIRECTORY_SEPARATOR . $class . '.php';
                      if (is_file($file)) {
                          require_once $file;
                          break;
                      }
                  }
              } else {
                  $file = $this->migrationPath . DIRECTORY_SEPARATOR . $class . '.php';
                  require_once $file;
              }
          }
      }
docs: Includes the migration file for a given migration class name.
  
  This function will do nothing on namespaced migrations, which are loaded by
  autoloading automatically. It will include the migration file, by searching
  [[migrationPath]] for classes without namespace.
  @param string $class the migration class name.
  @since 2.0.12