title: Required properties are undefined
body: <p>I was surprised to find out that TypeScript class properties can disappear from the transpiled output if they are never assigned a value.</p>
  
  <p>This TypeScript class...</p>
  
  <pre><code>class Foo {
      value: any;
  }
  </code></pre>
  
  <p>Becomes..</p>
  
  <pre><code>var Foo = (function () {
      function Foo() {
      }
      return Foo;
  }());
  </code></pre>
  
  <p>The <code>Foo.value</code> property is not declared as an optional property so I would expect the constructor to create that property, regardless of whether it has been assigned a value.</p>
  
  <pre><code>var Foo = (function () {
      function Foo() {
          Object.defineProperty(this, 'value', {});
      }
      return Foo;
  }());
  </code></pre>
  
  <p>How do I get that behavior?</p>