devDependencies: add core-js and es6-symbol polyfill for tests

package.json

@@ -29,9 +29,11 @@
     "browserify": "^12.0.1",
     "bundle-collapser": "^1.1.1",
     "coffee-script": "^1.8.0",
+    "core-js": "^2.2.1",
     "coveralls": "^2.11.6",
     "del": "^2.0.2",
     "derequire": "^2.0.3",
+    "es6-symbol": "^3.0.2",
     "eslint": "1.10.3",
     "eslint-plugin-react": "4.1.0",
     "eslint-plugin-react-internal": "file:eslint-rules",