message: Eliminate "An async function must return a (supertype of) Promise" errors related to async functions.
diff: @@ -38,7 +38,10 @@ const SecurityException = goog.require('tink.exception.SecurityException');
  * @final
  */
  class EciesAeadHkdfPrivateKeyFactory {
  - /** @override */
  + /**
  + * @override
  + * @return {!Promise<!PbMessage>}
  + */
  async newKey(keyFormat) {
  if (!keyFormat) {
  throw new SecurityException('Key format has to be non-null.');
  @@ -49,7 +52,10 @@ class EciesAeadHkdfPrivateKeyFactory {
  return await EciesAeadHkdfPrivateKeyFactory.newKeyImpl_(keyFormatProto);
  }
  - /** @override */
  + /**
  + * @override
  + * @return {!Promise<!PbKeyData>}
  + */
  async newKeyData(serializedKeyFormat) {
  const key = await this.newKey(serializedKeyFormat);
  