Polymer Unit Testing: Branch coverage and testing anonymous function within setInterval()

so I have this function within component:

    move: function() {
      var elemStyle = document.querySelector('#bar');
      var width = 1;
      var id = setInterval(function() {
        if (width >= 100) {
          clearInterval(id);
        } else {
          width++;
          elemStyle.style.width = width + '%';
        }
      }, 20);
    }

And I am writing tests to validate it. So far so good, but the only problems I am getting are the following:

1. It is saying that I am not Branch testing correctly (at 0%. which I assume has to do with the IF statement inside the setInterval anonymous function)

2. I have no idea how to test the anonymous function within setInterval, and much less the if statement.

I have succesfully tested the behaviour of the function itself (in this case, increase the width of a given element up to 100%), but I get this complaint about lack of branch testing. I have tried adding a return value after the clearInterval(id), but can't get it to work. What can I do? Any help will be appreciated.