Fix UI for retained size of a class.

runtime/bin/vmservice/observatory/lib/src/elements/class_view.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 class_view_element;
 
 import 'dart:async';
 import 'observatory_element.dart';
 import 'package:observatory/service.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('class-view')
 class ClassViewElement extends ObservatoryElement {
   @published Class cls;
   @observable ServiceMap instances;
   @observable int retainedBytes;
   ClassViewElement.created() : super.created();
 
   Future<ServiceObject> eval(String text) {
     return cls.get("eval?expr=${Uri.encodeComponent(text)}");
   }
 
   Future<ServiceObject> reachable(var limit) {
     return cls.get("instances?limit=$limit")
         .then((ServiceMap obj) {
           instances = obj;
         });
   }
-  
+
   // TODO(koda): Add no-arg "calculate-link" instead of reusing "eval-link".
   Future<ServiceObject> retainedSize(var dummy) {
-    return cls.get("retained")
-        .then((ServiceMap obj) {
-          retainedBytes = int.parse(obj['valueAsString']);
-        });
+    return cls.get("retained").then((Instance obj) {
+      retainedBytes = int.parse(obj.valueAsString);
+    });
   }
 
   void refresh(var done) {
     instances = null;
     retainedBytes = null;
     cls.reload().whenComplete(done);
   }
 
   void refreshCoverage(var done) {
     cls.refreshCoverage().whenComplete(done);
   }
 }