Async-step, first cut.

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';
@@ skipping 137 matching lines @@
   vm.getEventStream(streamId).then((stream) {
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
       handler(event, subscription, completer);
     });
   });
   return completer.future;
 }
 
 
-Future<Isolate> hasStoppedAtBreakpoint(Isolate isolate) async {
-  await isolate.reload();  // Might have missed pauseEvent.
-
-  if ((isolate.pauseEvent != null) &&
-      (isolate.pauseEvent.kind == ServiceEvent.kPauseBreakpoint)) {
-    // Already waiting at a breakpoint.
-    print('Breakpoint reached');
-    return new Future.value(isolate);
-  }
-
+Future<Isolate> hasStoppedAtBreakpoint(Isolate isolate) {

[Cutch, 2015/07/15 17:04:23]

why remove async?
why switch from using await to using then()?

   // Set up a listener to wait for breakpoint events.
   Completer completer = new Completer();
   isolate.vm.getEventStream(VM.kDebugStream).then((stream) {
     var subscription;
     subscription = stream.listen((ServiceEvent event) {
+        print("Event: $event");
         if (event.kind == ServiceEvent.kPauseBreakpoint) {
           print('Breakpoint reached');
           subscription.cancel();
-          completer.complete();
+          if (completer != null) {
+            completer.complete(isolate);
+            completer = null;
+          }
         }
     });
+
+    // Pause may have happened before we subscribed.
+    isolate.reload().then((_) {
+      if ((isolate.pauseEvent != null) &&
+         (isolate.pauseEvent.kind == ServiceEvent.kPauseBreakpoint)) {
+        // Already waiting at a breakpoint.
+        print('Breakpoint reached');
+        subscription.cancel();
+        if (completer != null) {
+          completer.complete(isolate);
+          completer = null;
+        }
+      }
+    });
   });
 
   return completer.future;  // Will complete when breakpoint hit.
 }
 
 
 Future<Isolate> resumeIsolate(Isolate isolate) {
   Completer completer = new Completer();
   isolate.vm.getEventStream(VM.kDebugStream).then((stream) {
     var subscription;
@@ skipping 60 matching lines @@
       }, onError: (e, st) {
         process.requestExit();
         if (!_isWebSocketDisconnect(e)) {
           print('Unexpected exception in service tests: $e $st');
           throw e;
         }
       });
     });
   }
 }