[HOTFIX] Flash message component stopped rendering
In Ember 1.12.0 and beyond, Ember.Component's access to template is being deprecated. In order to check if a block has been passed into the component, we can now use the hasBlock prop – see http://emberjs.com/api/classes/Ember.Component.html#property_hasBlock. As this is currently behind a feature flag that is false by default, this includes a polyfill to get it working as expected.