Update dependency core-js to v3

Author: renovate-bot

package.json

@@ -7,7 +7,7 @@
   },
   "dependencies": {
     "babel-runtime": "6.26.0",
-    "core-js": "2.5.7",
+    "core-js": "3.12.1",
     "meteor-node-stubs": "0.4.1",
     "react": "15.6.2",
     "react-dom": "15.6.2"