title: How to add `System.Web.Extensions` assembly to .net core project in vscode
body: <p>I have installed c# support for vscode(version is <code>1.15.0</code>) and created a HelloWorld project via <code>dotnet new console</code>. </p>
  
  <p>Now in <code>Program.cs</code>, I would like to use <code>JavaScriptSerializer</code> class which depends on <code>System.Web.Extensions</code> assembly. I typed <code>using System.Web.Script.Serialization;</code> and run <code>dotnet restore</code> ,but vscode cannot resolve it. The error is,  </p>
  
  <blockquote>
    <p>The type or namespace name 'Web' does not exist in the namespace
    'System' (are you missing an assembly reference?)</p>
  </blockquote>
  
  <p>It seems that <code>System.Web</code> is not part of .net core, but is there any way to add the assembly to the project?</p>
  
  <p>I cannot find a <code>project.json</code> file which is refered in other posts since it is a csproj project.</p>