Rework error handling in the service protocol and in Observatory.

runtime/observatory/tests/service/test_helper.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=--compile-all --error_on_bad_type --error_on_bad_override --checked
 
 library test_helper;
 
 import 'dart:async';
 import 'dart:convert';
 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.
 const String _TESTEE_MODE_FLAG = "--testee-mode";
 
 class _TestLauncher {
   Process process;
   final List<String> args;
   bool killedByTester = false;
 
@@ skipping 181 matching lines @@
       }, onError: (e, st) {
         process.requestExit();
         if (!_isWebSocketDisconnect(e)) {
           print('Unexpected exception in service tests: $e $st');
           throw e;
         }
       });
     });
   }
 }