Error if Symbol not defined.

[Joe Pea]

mixwith.js/src/mixwith.js

Line 181 in d34fe88

 if (Symbol && Symbol.hasInstance && !mixin[Symbol.hasInstance]) { 

Seems that if Symbol is not defined, there will be an error. Maybe it needs to be a typeof Symbol != 'undefined' check.

[Joe Pea]

Oh, and

mixin[Symbol.hasInstance]

will check for Function.prototype[Symbol.hasInstance]. Should that check for an own symbol property instead?