TypeScript compilation flaw

1 None-compiling TypeScript.ts

class AbstractProxy {
  private privateBuffer:String = 'abstract';
  
  public usePrivateBuffer = () => {
    alert(this.privateBuffer);
  }
}

/*
This doesn't compile
*/
class ProxyImplementation extends AbstractProxy {
  private privateBuffer:String;
}

2 Resulting JavaScript.js

/*
This is why it doesn't compile
*/
var AbstractProxy = (function () {
  function AbstractProxy() {
    var _this = this;
    this.privateBuffer = 'abstract'; //pollutes the "this" namespace
    this.usePrivateBuffer = function () {
      alert(_this.privateBuffer);
    };
  }
  return AbstractProxy;
})();

3 Corrected JavaScript.js

/*
This is what the compile should have done
*/
function AbstractProxy() {
  //private area
  var privateBuffer = 'abstract';
  function usePrivateBuffer() {
    alert(privateBuffer);
  }
  //public exports
  return {
    usePrivateBuffer: usePrivateBuffer
  };
}