code: function assertInstanceOf (constructor, msg) {
      if (msg) flag(this, 'message', msg);
      var name = _.getName(constructor);
      this.assert(
          flag(this, 'object') instanceof constructor
        , 'expected #{this} to be an instance of ' + name
        , 'expected #{this} to not be an instance of ' + name
      );
    }
docs: ### .instanceof(constructor)
  
  Asserts that the target is an instance of `constructor`.
  
  var Tea = function (name) { this.name = name; }
  , Chai = new Tea('chai');
  
  expect(Chai).to.be.an.instanceof(Tea);
  expect([ 1, 2, 3 ]).to.be.instanceof(Array);
  
  @name instanceof
  @param {Constructor} constructor
  @param {String} message _optional_
  @alias instanceOf
  @api public