title: Error using async and await with filereader
body: <p>I'm trying to read a file using FileReader:</p>
  
  <pre><code>async readFile(event: any) {
      var file = event.target.files[0];
      var data:string
      if (file) {   
          var reader:FileReader = new FileReader();
           reader.onload = async function (evt : FileReaderEvent) {
              data = await evt.target.result;
              console.log(evt.target.result);
  
          };
          console.log(file);
          console.log(data);
          await reader.readAsText(file);
          await this.processFileContent(data);
      }
   }
  </code></pre>
  
  <p>However, evt.target.result still gets printed after my console.log(file) call. </p>
  
  <p>Does anyone know how I can obtain the result of the file and pass it to my processFileContent function?</p>