importKey method
Implementation
Future<CryptoKey> importKey(KeyFormat format, dynamic keyData,
        dynamic algorithm, bool extractable, Iterable<KeyUsage> keyUsages) =>
    js_util.promiseToFuture(js_util.callMethod(this, 'importKey', [
      format.value,
      keyData,
      algorithm,
      extractable,
      keyUsages.map((e) => e.value)
    ]));