changeVersion method Null safety

Implementation

@JSName('changeVersion')
/**
 * Atomically update the database version to [newVersion], asynchronously
 * running [callback] on the [SqlTransaction] representing this
 * [changeVersion] transaction.
 *
 * If [callback] runs successfully, then [successCallback] is called.
 * Otherwise, [errorCallback] is called.
 *
 * [oldVersion] should match the database's current [version] exactly.
 *
 * See also:
 *
 * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.
 */
Future<SqlTransaction> changeVersion(String oldVersion, String newVersion) {
  var completer = new Completer<SqlTransaction>();
  _changeVersion(oldVersion, newVersion, (value) {
    completer.complete(value);
  }, (error) {
    completer.completeError(error);
  });
  return completer.future;
}