title: instanceof check on interface
body: <p>Given the following code:</p>
  
  <pre><code>module MyModule {
    export interface IMyInterface {}
    export interface IMyInterfaceA extends IMyInterface {}
    export interface IMyInterfaceB extends IMyInterface {}
  
    function(my: IMyInterface): void {
      if (my instanceof IMyInterfaceA) {
        // do something cool
      }
    }
  }
  </code></pre>
  
  <p>I get the error "Can't find name IMyInterfaceA". What's the correct way to handle this situation?</p>