Fix observatory warnings

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

 // Copyright (c) 2014, 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_tree_element;
 
 import 'observatory_element.dart';
+import 'dart:async';
 import 'dart:html';
 import 'package:logging/logging.dart';
 import 'package:observatory/app.dart';
 import 'package:observatory/service.dart';
 import 'package:polymer/polymer.dart';
 
 class ClassTreeRow extends TableTreeRow {
   @reflectable final Isolate isolate;
   @reflectable final Class cls;
   ClassTreeRow(this.isolate, this.cls, TableTree tree, ClassTreeRow parent)
       : super(tree, parent) {
     assert(isolate != null);
     assert(cls != null);
   }
 
   void _addChildren(List<Class> subclasses) {
     for (var subclass in subclasses) {
       if (subclass.isPatch) {
         continue;
       }
       if (subclass.mixin != null) {
         _addChildren(subclass.subclasses);
       } else {
         var row = new ClassTreeRow(isolate, subclass, tree, this);
         children.add(row);
       }
     }
   }
 
-  void _addMixins(Class cls) async {
+  Future _addMixins(Class cls) async {
     var classCell = flexColumns[0];
     if (cls.superclass == null) {
       return;
     }
     bool first = true;
     while (cls.superclass != null && cls.superclass.mixin != null) {
       cls = cls.superclass;
       await cls.mixin.load();
       var span = new SpanElement();
       span.style.alignSelf = 'center';
       span.style.whiteSpace = 'pre';
       if (first) {
         span.text = ' with ';
       } else {
         span.text = ', ';
       }
       classCell.children.add(span);
       var mixinRef = new Element.tag('class-ref');
       mixinRef.ref = cls.mixin.typeClass;
       mixinRef.style.alignSelf = 'center';
       classCell.children.add(mixinRef);
       first = false;
     }
   }
 
-  void _addClass(Class cls) async {
+  Future _addClass(Class cls) async {
     var classCell = flexColumns[0];
     classCell.style.justifyContent = 'flex-start';
     var classRef = new Element.tag('class-ref');
     classRef.ref = cls;
     classRef.style.alignSelf = 'center';
     classCell.children.add(classRef);
     if (cls.superclass != null && cls.superclass.mixin != null) {
       await _addMixins(cls);
     }
     if (cls.subclasses.isNotEmpty) {
@@ skipping 69 matching lines @@
     } catch (e, stackTrace) {
       Logger.root.warning('_update', e, stackTrace);
     }
     // Check if we only have one node at the root and expand it.
     if (tree.rows.length == 1) {
       tree.toggle(tree.rows[0]);
     }
     notifyPropertyChange(#tree, null, tree);
   }
 }