message: AppTile: Test if widgetUrl startsWith instead of has a substring
  The event origin should be at the beginning of the URL.
diff: @@ -143,7 +143,7 @@ export default React.createClass({
  event.origin = event.originalEvent.origin;
  }
  - if (this.state.widgetUrl.indexOf(event.origin) === -1) {
  + if (!this.state.widgetUrl.startsWith(event.origin)) {
  return;
  }
  