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

runtime/observatory/lib/src/elements/curly_block.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.
 
 library curly_block_element;
 
 import 'package:polymer/polymer.dart';
+import 'observatory_element.dart';
 
 @CustomTag('curly-block')
-class CurlyBlockElement extends PolymerElement {
+class CurlyBlockElement extends ObservatoryElement {
   CurlyBlockElement.created() : super.created();
 
   @observable bool expanded = false;
   @observable bool busy = false;
   @published var callback = null;
   @published bool expand = false;
+  @published String expandKey;
 
   void expandChanged(oldValue) {
     expanded = expand;
   }
 
+  void expandKeyChanged(oldValue) {
+    if (expandKey != null) {
+      var value = app.expansions[expandKey];
+      if (value != null) {
+        if (expanded != value) {
+          toggleExpand(null, null, null);
+        }
+      }
+    }
+  }
+
   void doneCallback() {
     expanded = !expanded;
+    if (expandKey != null) {
+      app.expansions[expandKey] = expanded;
+    }
     busy = false;
   }
 
   void toggleExpand(var event, var b, var c) {
     assert(callback == null || expand == false);
     if (busy) {
       return;
     }
+    busy = true;
     if (callback != null) {
-      busy = true;
       callback(!expanded, doneCallback);
     } else {
-      expanded = !expanded;
+      doneCallback();
     }
   }
 }