title: Why do WinRT types have to be sealed?
body: <p>In several places (e.g. "<a href="http://msdn.microsoft.com/en-us/library/windows/apps/br230301%28v=VS.85%29.aspx#DiveRightIn">Creating Windows Runtime Components for JavaScript, in C# and Visual Basic</a>" on MSDN), I've seen it specified that, if you write a class in .NET that you want to use from JavaScript, then you must make it a sealed class.</p>
  
  <p>This seems like an arbitrary restriction. Why can JavaScript only work with sealed classes?</p>