Fix async_generator_breakpoint_test so it awaits on isolate resume requests

runtime/observatory/tests/service/async_generator_breakpoint_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 --verbose-debug
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 printSync() {  // Line 10
@@ skipping 57 matching lines @@
       .then((Instance result) {
         expect(result.valueAsString, equals('true'));
       });
 
   var stream = await isolate.vm.getEventStream(VM.kDebugStream);
   await for (ServiceEvent event in stream) {
     if (event.kind == ServiceEvent.kPauseBreakpoint) {
       var bp = event.breakpoint;
       print('Hit $bp');
       hits.add(bp);
-      isolate.resume();
+      await isolate.resume();
 
       if (hits.length == 5) break;
     }
   }
 
   expect(hits, equals([bp1, bp5, bp4, bp2, bp3]));
 }
 
 var tests = [testAsync];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testeeDo);