Patched isolate pause flag detection for newer VM versions

Newer observatory versions return the following block for paused isolates when querying their status:

```json
{
  "pauseEvent":{
     "type":"DebuggerEvent"
     ,"id":""
     ,"eventType":"IsolateShutdown"
}
```

With this patch, ```collect_coverage.dart``` will not hang indefinitely when invoked with ```--wait-paused```