Fix time span in profiler

runtime/observatory/lib/src/elements/cpu_profile.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 cpu_profile_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/service.dart';
@@ skipping 482 matching lines @@
     this.stackTrace = stackTrace;
     _stopWatch.reset();
     return window.animationFrame;
   }
 
   _update(CpuProfile sampleBuffer) {
     sampleCount = profile.sampleCount.toString();
     refreshTime = new DateTime.now().toString();
     stackDepth = profile.stackDepth.toString();
     sampleRate = profile.sampleRate.toStringAsFixed(0);
-    timeSpan = formatTime(profile.timeSpan);
+    if (profile.sampleCount == 0) {
+      timeSpan = '0s';
+    } else {
+      timeSpan = formatTime(profile.timeSpan);
+    }
   }
 
   void tagSelectorChanged(oldValue) {
     reload(profile.isolate);
   }
 
   Function onSampleBufferUpdate;
   @observable bool showTagSelector = true;
 
   @observable String sampleCount = '';
@@ skipping 663 matching lines @@
       return;
     }
     var tree = profile.loadCodeTree(exclusive ? 'exclusive' : 'inclusive');
     if (tree == null) {
       return;
     }
     var rootRow = new CodeProfileTreeRow(codeTree, null, profile, tree.root);
     codeTree.initialize(rootRow);
   }
 }