title: Sourcemap source files are empty using gulp-uglifyjs
body: <p>Looks like the source map files are blank.</p>
  
  <p>My code is as follows. </p>
  
  <pre><code>gulp.src(sources)
      .pipe(uglifyjs('app.min.js', {
          outSourceMap: true
      }))
      .pipe(gulp.dest('public/js'))
      .pipe(notify({message: 'Scripts task complete'}));
  </code></pre>
  
  <p>in the browser my image looks like this </p>
  
  <p><img src="https://imagizer.imageshack.us/v2/640x480q90/661/4MmFXo.png" alt=""></p>
  
  <p>Notice how the source files are blank. Not sure what I am doing wrong here. Can you please help.</p>
  
  <p>I will add that my source files are using the module pattern i.e.</p>
  
  <pre><code>(function () {
  "use strict";
  //my code in here
  }())
  </code></pre>