old_contents: (function() {
    'use strict';
    if ('padStart' in String.prototype) return;
    String.prototype.padStart = function(len, str) {
      str = str === undefined ? '\u0020' : String(str);
      if (len <= this.length || str.length === 0) return String(this);
      var buffer = new Array(len-this.length);
      for (var i = 0; i < buffer.length; i++) buffer[i] = str[i%str.length];
      return buffer.join('').concat(this);
    };
  })();
  
new_contents: (function() {
    'use strict';
    if ('padStart' in String.prototype) return;
    String.prototype.padStart = function(len, str) {
      len = parseInt(len); if (isNaN(len)) len = 0;
      str = str === undefined ? '\u0020' : String(str);
      if (len <= this.length || str.length === 0) return String(this);
      var buffer = new Array(len-this.length);
      for (var i = 0; i < buffer.length; i++) buffer[i] = str[i%str.length];
      return buffer.join('').concat(this);
    };
  })();
  
subject: Fix len parameter