http2 hung at await for stream.incomingMessages

in http2 demo there is such code to deal message;
```
var stream = transport.makeRequest(headers, endStream: true);
await for (var message in stream.incomingMessages) {
```

but if the connection is closed after makeRequest, then the await will hung there;

Start with the HTTP/2 demo and trace the code around transport.makeRequest and stream.incomingMessages. Reproduce the case where the connection closes immediately after makeRequest, then identify the relevant stream-termination behavior; done should include a test or demonstrated behavior showing that awaiting incomingMessages completes instead of hanging.

Written by the indexing model from the issue text.