Allow users to see more elements of a list/map in Observatory.

runtime/observatory/lib/src/elements/instance_ref.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 instance_ref_element;
 
+import 'dart:async';
 import 'package:polymer/polymer.dart';
 import 'package:observatory/service.dart';
 import 'service_ref.dart';
 
 @CustomTag('instance-ref')
 class InstanceRefElement extends ServiceRefElement {
   InstanceRefElement.created() : super.created();
 
   String get hoverText {
     if (ref != null) {
@@ skipping 34 matching lines @@
       Instance refMap = ref;
       refMap.fields = null;
       refMap.elements = null;
       onDone();
     }
   }
 
   String makeExpandKey(String key) {
     return '${expandKey}/${key}';
   }
+
+  Future showMore() async {
+    Instance instance = ref;
+    if (instance.isList) {
+      await instance.reload(count: instance.elements.length * 2);
+    } else if (instance.isMap) {
+      await instance.reload(count: instance.associations.length * 2);
+    } else if (instance.isTypedData) {
+      await instance.reload(count: instance.typedElements.length * 2);
+    }
+  }
 }