remove core-js warnings

Co-authored-by: Leonardo Dino <[email protected]>

Author: ChristopherBiscardi

packages/gatsby-mdx/package.json

@@ -12,7 +12,10 @@
     "@mdx-js/react": "^1.0.0-alpha.17"
   },
   "dependencies": {
+    "@babel/core": "^7.4.0",
     "@babel/plugin-proposal-object-rest-spread": "^7.0.0",
+    "@babel/preset-env": "^7.4.2",
+    "core-js": "2",
     "debug": "^4.0.1",
     "escape-string-regexp": "^1.0.5",
     "fs-extra": "^7.0.0",

packages/gatsby-mdx/utils/gen-mdx.js

@@ -138,6 +138,7 @@ ${code}`;
           require("@babel/preset-env"),
           {
             useBuiltIns: "entry",
+            corejs: 2,
             modules: "false"
           }
         ]