isLocalUri example

  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    // If we're stubbing a file entity, return a URI of NULL so it will get     // stubbed by the general process.     if ($row->isStub()) {
      return NULL;
    }
    [$source, $destination] = $value;

    // If the source path or URI represents a remote resource, delegate to the     // download plugin.     if (!$this->isLocalUri($source)) {
      return $this->downloadPlugin->transform($value, $migrate_executable, $row, $destination_property);
    }

    // Ensure the source file exists, if it's a local URI or path.     if (!file_exists($source)) {
      throw new MigrateException("File '$source' does not exist");
    }

    // If the start and end file is exactly the same, there is nothing to do.     if ($this->isLocationUnchanged($source, $destination)) {
      return $destination;
    }