Fix checked mode failure of causal_async_star_stack_contents test

runtime/observatory/lib/src/service/object.dart

 // Copyright (c) 2014, 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.
 
 part of service;
 
 // Some value smaller than the object ring, so requesting a large array
 // doesn't result in an expired ref because the elements lapped it in the
 // object ring.
 const int kDefaultFieldLimit = 100;
@@ skipping 4386 matching lines @@
     description = map['description'];
     vmName = map['name'];
     value = map['value'];
     min = map['min'];
     max = map['max'];
   }
 
   String toString() => "ServiceMetric($_id)";
 }
 
-void printFrames(List<Frame> frames) async {
+Future<Null> printFrames(List<Frame> frames) async {
   for (int i = 0; i < frames.length; i++) {
     final Frame frame = frames[i];
     String frameText = await frame.toUserString();
     print('#${i.toString().padLeft(3)}: $frameText');
   }
 }
 
 class Frame extends ServiceObject implements M.Frame {
   M.FrameKind kind = M.FrameKind.regular;
   int index;
@@ skipping 34 matching lines @@
   String toString() {
     if (function != null) {
       return "Frame([$kind] ${function.qualifiedName} $location)";
     } else if (location != null) {
       return "Frame([$kind] $location)";
     } else {
       return "Frame([$kind])";
     }
   }
 
-  String toUserString() async {
+  Future<String> toUserString() async {
     if (function != null) {
       return "Frame([$kind] ${function.qualifiedName} "
              "${await location.toUserString()})";
     } else if (location != null) {
       return "Frame([$kind] ${await location.toUserString()}";
     } else {
       return "Frame([$kind])";
     }
   }
 }
@@ skipping 100 matching lines @@
     var v = list[i];
     if ((v is Map) && _isServiceMap(v)) {
       list[i] = owner.getFromMap(v);
     } else if (v is List) {
       _upgradeList(v, owner);
     } else if (v is Map) {
       _upgradeMap(v, owner);
     }
   }
 }