title: Check if variable is a specific interface type in a typescript union
body: <p>Is it possible to create a typeguard, or something else that accomplishes the same purpose, to check if a variable is a specific interface type in a typescript union?</p>
  
  <pre><code>interface Foo { a:string }
  interface Bar { b:string }
  
  (function() {
      function doStuff(thing: Foo | Bar) {
          if(typeof thing === 'Foo') {
              console.log('Foo');
          } 
          else if (typeof thing === 'Bar') {
              console.log('Bar');
          }
          else {
              console.log('unknown');
          }
      }
  
      var thing: Foo = {a:'a'};
      doStuff(thing);
  })();
  </code></pre>