Add missing MissingElement import in index.js

Closes #290

Author: marcoroth

javascript/index.js

@@ -12,6 +12,7 @@ import OperationStore, { addOperation, addOperations } from './operation_store'
 import StreamFromElement from './elements/stream_from_element'
 import UpdatesForElement from './elements/updates_for_element'
 import SubscribingElement from './elements/subscribing_element'
+import MissingElement from './missing_element'
 import CableConsumer from './cable_consumer'
 import Debug from './debug'