Fixed crash during retainedTopList computation

runtime/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 'sample_buffer_control.dart';
 import 'stack_trace_tree_config.dart';
@@ skipping 23 matching lines @@
   Future<ServiceObject> evaluate(String expression) {
     return cls.evaluate(expression);
   }
 
   Future<ServiceObject> reachable(var limit) {
     return cls.isolate.getInstances(cls, limit).then((ServiceMap obj) {
       instances = obj;
     });
   }
 
-  Future<ServiceObject> retainedToplist(var limit) {
-    return cls.isolate.fetchHeapSnapshot(true).last
-      .then((RawHeapSnapshot raw) async {
-        final snapshot = new HeapSnapshot();
-        await snapshot.loadProgress(cls.isolate, raw);
-        return snapshot;
-      })
-      .then((HeapSnapshot snapshot) =>
-          Future.wait(snapshot.getMostRetained(cls.isolate, classId: cls.vmCid,
-                                               limit: 10)))
-      .then((List<ServiceObject> most) {
-        mostRetained = new ObservableList.from(most);
-      });
+  Future retainedToplist(var limit) async {
+      final raw = await cls.isolate.fetchHeapSnapshot(true).last;
+      final snapshot = new HeapSnapshot();
+      await snapshot.loadProgress(cls.isolate, raw).last;
+      final most = await Future.wait(snapshot.getMostRetained(cls.isolate,
+                                                              classId: cls.vmCid,
+                                                              limit: 10));
+      mostRetained = new ObservableList.from(most);
   }
 
   // TODO(koda): Add no-arg "calculate-link" instead of reusing "eval-link".
   Future<ServiceObject> reachableSize(var dummy) {
     return cls.isolate.getReachableSize(cls).then((Instance obj) {
       reachableBytes = int.parse(obj.valueAsString);
     });
   }
 
   Future<ServiceObject> retainedSize(var dummy) {
@@ skipping 56 matching lines @@
   Future toggleAllocationTrace() {
     if (cls == null) {
       return new Future(refresh);
     }
     if (cls.traceAllocations) {
       refreshAllocationProfile();
     }
     return cls.setTraceAllocations(!cls.traceAllocations).whenComplete(refresh);
   }
 }