Fix flow definitions for auth.js

Author: BlooJeans

Diff for: lib/modules/auth.js

@@ -138,15 +138,15 @@ export default class Auth extends Base {
    * Sign the user in with a third-party authentication provider
    * @return {Promise}           A promise resolved upon completion
    */
-  signInWithCredential(credential): Promise<Object> {
+  signInWithCredential(credential: Object): Promise<Object> {
     return promisify('signInWithProvider', FirestackAuth)(credential.provider, credential.token, credential.secret);
   }
 
   /**
    * Re-authenticate a user with a third-party authentication provider
    * @return {Promise}         A promise resolved upon completion
    */
-  reauthenticateUser(credential): Promise<Object> {
+  reauthenticateUser(credential: Object): Promise<Object> {
     return promisify('reauthenticateWithCredentialForProvider', FirestackAuth)(credential.provider, credential.token, credential.secret);
   }