added rest args to ES6 canary to prevent test execution on Node.js@5.12

Author: mikesamuel

test/unit/es6/canary.js

@@ -1,3 +1,7 @@
 // A minimal file that uses ES6 features which will fail to load on
 // older browsers.
 `I load on ES6`;
+
+module.exports = function usesRestArgs (...args) {
+  return args;
+};