Rework error handling in the service protocol and in Observatory.

runtime/observatory/tests/service/test_helper.dart

Index: runtime/observatory/tests/service/test_helper.dart
diff --git a/runtime/observatory/tests/service/test_helper.dart b/runtime/observatory/tests/service/test_helper.dart
index fd639339c41e9fc74aa99ab03685e4c671d309bd..6da69becfc01f52b5f273e55768acc87ae72a345 100644
--- a/runtime/observatory/tests/service/test_helper.dart
+++ b/runtime/observatory/tests/service/test_helper.dart
@@ -11,10 +11,7 @@ import 'dart:io';
 import 'package:observatory/service_io.dart';
 
 bool _isWebSocketDisconnect(e) {
-  if (e is! ServiceException) {
-    return false;
-  }
-  return (e as ServiceException).message == 'WebSocket closed';
+  return e is NetworkRpcException;
 }
 
 // This invocation should set up the state being tested.