Add navigateFallbackWhitelist /__* to SW Precache config (facebook#2347)

* Add navigateFallbackWhitelist /__* to SW Precache config

* Add comments

Author: ryansully

packages/react-scripts/config/webpack.config.prod.js

@@ -319,7 +319,12 @@ module.exports = {
         console.log(message);
       },
       minify: true,
+      // For unknown URLs, fallback to the index page
       navigateFallback: publicUrl + '/index.html',
+      // Ignores URLs starting from /__ (useful for Firebase):
+      // https://github.com/facebookincubator/create-react-app/issues/2237#issuecomment-302693219
+      navigateFallbackWhitelist: [/^(?!\/__).*/],
+      // Don't precache sourcemaps (they're large) and build asset manifest:
       staticFileGlobsIgnorePatterns: [/\.map$/, /asset-manifest\.json$/],
       // Work around Windows path issue in SWPrecacheWebpackPlugin:
       // https://github.com/facebookincubator/create-react-app/issues/2235