title: Why do I get "TypeError: property of '__source' is undefined : object is not extensible." error in IE11?
body: <p>My code works fine in Chrome and Firefox, but I get that error in IE 11. 
  The full error:</p>
  
  <blockquote>
    <p>Attempting to configure '__source' with descriptor '{"value":"DefaultValueAccessor","configurable":true}' on object 'function ElementRef(nativeElement) {
                this.nativeElement = nativeElement;
            }' and got error, giving up: TypeError: property of '__source' is undefined : object is not extensible.</p>
  </blockquote>
  
  <p>My code:</p>
  
  <pre><code>for(let x = 0; x &lt; this.files.length; x++) {
  
        let filename = this.files[x].name.slice((Math.max(0, this.files[x].name.lastIndexOf(".")) || Infinity) + 1);
  
        for(let y = 0; y &lt; acceptedFileExtensions.length; y ++) {
          if(filename === acceptedFileExtensions[y]) {
            this.supportedSelectedFiles.push(this.files[x]);
            formData.append('files', this.files[x]);
          }
        }
      }
  </code></pre>
  
  <p>Then I upload the form data with an HTTP post to my server. What is the solution to make this work in IE 11 too?</p>