code: function(expr, msg, negatedMsg, expected, showDiff){
          var msg = this.negate ? negatedMsg : msg
              , ok = this.negate ? !expr : expr
              , obj = this.obj;
  
          if (ok) return;
  
          var err = new AssertionError({
              message: msg.call(this)
              , actual: obj
              , expected: expected
              , stackStartFunction: this.assert
              , negated: this.negate
          });
  
          err.showDiff = showDiff;
  
          throw err;
      }
docs: Assert _expr_ with the given _msg_ and _negatedMsg_.
  
  @param {Boolean} expr
  @param {String} msg
  @param {String} negatedMsg
  @param {Object} expected
  @api private