Add ES2017 to package.json

Author: hsubox76

packages/webchannel-wrapper/package.json

@@ -5,6 +5,7 @@
   "author": "Firebase <[email protected]> (https://firebase.google.com/)",
   "main": "dist/index.js",
   "module": "dist/index.esm.js",
+  "esm2017": "dist/index.esm2017.js",
   "files": [
     "dist"
   ],