Remember when a variable has been expanded/unexpanded in the debugger.

runtime/observatory/lib/src/elements/instance_ref.dart

Index: runtime/observatory/lib/src/elements/instance_ref.dart
diff --git a/runtime/observatory/lib/src/elements/instance_ref.dart b/runtime/observatory/lib/src/elements/instance_ref.dart
index 19a87e1ee68eccbcf072c4effaeb3c041e35b2da..86727ae8d36b07448cf08caa438fe8b4bec83b47 100644
--- a/runtime/observatory/lib/src/elements/instance_ref.dart
+++ b/runtime/observatory/lib/src/elements/instance_ref.dart
@@ -54,4 +54,8 @@ class InstanceRefElement extends ServiceRefElement {
       onDone();
     }
   }
+
+  String makeExpandKey(String key) {
+    return '${expandKey}/${key}';
+  }
 }