Add tests of line numbers to causal async stack tests

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.
 
 library test_helper;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'package:observatory/service_io.dart';
@@ skipping 312 matching lines @@
             testIndex++;
             await test(isolate);
           }
         }
 
         print('All service tests completed successfully.');
         testsDone = true;
         await process.requestExit();
       });
     }, onError: (error, stackTrace) async {
-      print('onERROR FIRED!');
       if (testsDone) {
         print('Ignoring late exception during process exit:\n'
               '$error\n#stackTrace');
       } else {
         await process.requestExit();
         print('Unexpected exception in service tests: $error\n$stackTrace');
         throw error;
       }
     });
   }
@@ skipping 135 matching lines @@
     new _ServiceTesterRunner().run(
         mainArgs: mainArgs,
         extraArgs: extraArgs,
         vmTests: tests,
         pause_on_start: pause_on_start,
         pause_on_exit: pause_on_exit,
         verbose_vm: verbose_vm,
         pause_on_unhandled_exceptions: pause_on_unhandled_exceptions);
   }
 }