isArray is not using the native isArray method, causing incompatibilities with prototype.js

[mhariri]

isArray should use the browser's native implementation of isArray if available. This causes errors when running regenerate together with prototype.js:

symbol.charCodeAt is not a function
	TypeError: symbol.charCodeAt is not a function
	    at symbolToCodePoint (http://localhost:1090/IPdeploy/js/systemjs-babel-browser.js:24493:23)
	    at regenerate.add (http://localhost:1090/IPdeploy/js/systemjs-babel-browser.js:24928:64)
	    at regenerate (http://localhost:1090/IPdeploy/js/systemjs-babel-browser.js:24902:28)
	    at eval (http://localhost:1090/IPdeploy/js/systemjs-babel-browser.js:25033:8)