title: Return Task<bool> instantly
body: <p>I have a list of tasks, which i'd like to wait for. I'm waiting like </p>
  
  <pre><code>    await TaskEx.WhenAll(MyViewModel.GetListOfTasks().ToArray());
  </code></pre>
  
  <p>MyViewModel.GetListOfTasks() returns List of Task:</p>
  
  <pre><code>    var tasksList = new List&lt;Task&gt;();
      foreach (var item in Items)
      {                
          tasksList.Add(item.MyTask());
      }
  </code></pre>
  
  <p>Now, i'd like to return dummy task, which would be done immediately. However, TaskEx.WhenAll would wait it forever:</p>
  
  <pre><code>    public Task&lt;bool&gt; MyTask()
      {
          return new Task&lt;bool&gt;(() =&gt; false);
      }
  </code></pre>
  
  <p>How can i return Task, which would be finished instantly?</p>