Rewrite asyncStepOver to use await

runtime/observatory/lib/src/elements/debugger.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 debugger_page_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:math';
 import 'observatory_element.dart';
@@ skipping 1808 matching lines @@
       console.print('The program is already running');
     }
   }
 
   Future asyncNext() async {
     if (isolatePaused()) {
       var event = isolate.pauseEvent;
       if (event.asyncContinuation == null) {
         console.print("No async continuation at this location");
       } else {
-        return isolate.asyncStepOver()[Isolate.kFirstResume];
+        List<Future> asyncStepFutures = await isolate.asyncStepOver();
+        return asyncStepFutures[Isolate.kFirstResume];
       }
     } else {
       console.print('The program is already running');
     }
   }
 
   Future syncNext() async {
     if (isolatePaused()) {
       var event = isolate.pauseEvent;
       if (event.kind == ServiceEvent.kPauseStart) {
@@ skipping 813 matching lines @@
         }
       });
   }
 
   void focus() {
     $['textBox'].focus();
   }
 
   DebuggerInputElement.created() : super.created();
 }