Observatory: Fix instance view of lists.

runtime/observatory/lib/src/elements/helpers/any_ref.dart

Index: runtime/observatory/lib/src/elements/helpers/any_ref.dart
diff --git a/runtime/observatory/lib/src/elements/helpers/any_ref.dart b/runtime/observatory/lib/src/elements/helpers/any_ref.dart
index 704e1a5c990d669f12a9cc00913c0545d6d2fe0d..a15133abc222e5e499ee531e6b5d290e8d4d71c6 100644
--- a/runtime/observatory/lib/src/elements/helpers/any_ref.dart
+++ b/runtime/observatory/lib/src/elements/helpers/any_ref.dart
@@ -28,8 +28,11 @@ import 'package:observatory/src/elements/unknown_ref.dart';
 Element anyRef(M.IsolateRef isolate, ref,
     M.InstanceRepository instances, {RenderingQueue queue}) {
   if (ref is M.Guarded) {
-    return anyRef(isolate, ref.asSentinel ?? ref.asValue, instances,
-        queue: queue);
+    if (ref.isSentinel) {
+      return anyRef(isolate, ref.asSentinel, instances, queue: queue);
+    } else {
+      return anyRef(isolate, ref.asValue, instances, queue: queue);
+    }
   } else if (ref is M.ObjectRef) {
     if (ref is M.ClassRef) {
       return new ClassRefElement(isolate, ref, queue: queue);
@@ -72,5 +75,5 @@ Element anyRef(M.IsolateRef isolate, ref,
   } else if (ref is M.Sentinel) {
     return new SentinelValueElement(ref, queue: queue);
   }
-  throw new Exception('Unknown runtimeType (${ref.runtimeType})');
+  throw new Exception('Unknown ref type (${ref.runtimeType})');
 }