Make resolve return a Promise (#743)

Fixes #742

Author: npm1

spec/index.bs

@@ -1715,7 +1715,7 @@ This specification introduces the {{IdentityUserInfo}} dictionary as well as the
 
   [Exposed=Window, SecureContext] interface IdentityProvider {
       static undefined close();
-      static undefined resolve(DOMString token, optional IdentityResolveOptions options = {});
+      static Promise&ltundefined> resolve(DOMString token, optional IdentityResolveOptions options = {});
       static Promise<sequence<IdentityUserInfo>> getUserInfo(IdentityProviderConfig config);
   };
 </pre>