[Bsb] Revert the React template from es6 to commonjs (#2617)

* [Bsb] Revert the React template from es6 to commonjs

See #2587 (comment)

* Update README.md

Author: chenglou

jscomp/bsb/templates/react/README.md

@@ -20,3 +20,5 @@ npm run webpack:production
 ```
 
 This will replace the development artifact `build/Index.js` for an optimized version.
+
+**To enable dead code elimination**, change `bsconfig.json`'s `package-specs` `module` from `"commonjs"` to `"es6"`. Then re-run the above 2 commands. This will allow Webpack to remove unused code.

jscomp/bsb/templates/react/bsconfig.json

@@ -12,7 +12,7 @@
     "subdirs" : true
   },
   "package-specs": [{
-    "module": "es6",
+    "module": "commonjs",
     "in-source": true
   }],
   "suffix": ".bs.js",