Display "Loading..." on Debug page after an `IsolateDestroy` event

For large applications, creating an isolate can take non-trivial time. Currently DevTools displays `Loading...` while it waits for the initial `getVM` response. This is a nice user experience while `package:dwds` creates the first isolate. Upon a refresh in which `package:dwds` sends an `IsolateDestroy` event, DevTools will properly clear out the content of the Debug page. However, it does not display any indication that it is waiting for an `IsolateCreate` event. Ideally DevTools displays the `Loading...` message again.