If testeeBefore returns a future, wait for it.

runtime/observatory/tests/service/pause_on_unhandled_exceptions_test.dart

 // Copyright (c) 2015, 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=--error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:async';
 
 doThrow() {
   throw "TheException"; // Line 13.
   return "end of doThrow";
 }
 
 var tests = [
   hasStoppedWithUnhandledException,
+
   (Isolate isolate) async {
+    print("We stoppped!");
     var stack = await isolate.getStack();
     expect(stack['frames'][0].function.name, equals('doThrow'));
   }
 ];
 
-main(args) => runIsolateTests(args,
-                              tests,
-                              pause_on_unhandled_exceptions: true,
-                              testeeConcurrent: doThrow);
+main(args) => runIsolateTestsSynchronous(args,
+                                         tests,
+                                         pause_on_unhandled_exceptions: true,
+                                         testeeConcurrent: doThrow);