ensure single package for ui-keyring (#120)

packages/ui-keyring/src/index.ts

@@ -2,8 +2,12 @@
 // This software may be modified and distributed under the terms
 // of the Apache-2.0 license. See the LICENSE file for details.
 
+import { assertSingletonPackage } from '@polkadot/util';
+
 import keyring, { Keyring } from './Keyring';
 
+assertSingletonPackage('@polkadot/ui-keyring');
+
 export default keyring;
 
 export {