title: The type 'System.Uri' is defined in an assembly that is not referenced.What is this error?
body: <p>This question may already have an answer here:
  I am creating MvvmCross PCL library using Visual Studio 2013 ultimate on my windows 8 machine.</p>
  
  <p>And My PCL target frameworks are -</p>
  
  <ul>
  <li>Net framework 4.52 and higher</li>
  <li>W 8</li>
  <li>xamarin Andoid</li>
  <li>xamarin iOs</li>
  </ul>
  
  <p>I am getting following error
  <strong>The type 'System.Uri' is defined in an assembly that is not referenced. You must add a reference to assembly 'System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089.</strong></p>
  
  <p>I am creating PCL using MvvmCross.One method in my FirstViewModel class is as follows.</p>
  
  <pre><code>    public async Task&lt;object&gt; GetConstituencyByUserId(string userId)
      {
          try
          {
                HttpClient _client = new HttpClient();
                string url ="";
                HttpResponseMessage response = await _client.GetAsync(url);
                if (response.ReasonPhrase.Contains("OK"))
                {
                   if (response != null)
                   {
                       var jsonSerializer = 
                       new DataContractJsonSerializer(typeof(constituency));
                       var stream = await     response.Content.ReadAsStreamAsync();
                       return (constituency)jsonSerializer.ReadObject(stream);
                   }
  
                }
                else if (response.ReasonPhrase.Contains("Bad Request"))
                {
                   return null;
                }
                else
                {
                    return null;
                }
           }
           catch (Exception ex)
           {
                            // App.errorlog.WriteToLog(ex.Message);
           }
          return null;
      }
  </code></pre>
  
  <p>And It is pointing to line of HttpResponseMessage. and giving above error.</p>
  
  <p>Anyone knows ?</p>