Symbol check in lib/keys doesn't work with es6-shim

[wkwiatek]

es6-shim initializes Symbol as an object:
https://github.com/paulmillr/es6-shim/blob/master/es6-shim.js#L200

which produces error when trying to do following line: https://github.com/angular/zone.js/blob/master/lib/keys.js#L9

The simple solution for that would be to check if the Symbol is a function but maybe there's a better solution?