es2015/DBFactory.js
function executeMigrationListeners(databaseName, oldVersion, newVersion,
    completionPromise) {
  for (let listener of migrationListeners) {
    try {
      listener(databaseName, oldVersion, newVersion, completionPromise)
    } catch (e) {
      console.warn("A schema migration event listener threw an error", e);
    }
  }
}
function executeMigrationListeners(databaseName, oldVersion, newVersion,
    completionPromise) {
  for (let listener of migrationListeners) {
    try {
      listener(databaseName, oldVersion, newVersion, completionPromise)
    } catch (e) {
      console.warn("A schema migration event listener threw an error", e);
    }
  }
}
Executes the currently registered database schema migration listeners with
the provided arguments.

@param {string} databaseName The name of the database being migrated.
@param {number} oldVersion The version number from which the schema is being
migrated.
@param {number} newVersion The version number to which the schema is being
migrated.
@param {Promise<undefined>} completionPromise A promise that resolves when
the migration is finished.