Firebase Hosting with Polymer: Rewrite doesn't work as expected

Question

I have a polymer starter kit app and added an extra static html file called extra.html.

I followed the rewrite rule as advised in polymer docs and was expecting that if I navigate to this url , it will display extra.html. However, it's always fetching index.html instead.

My firebase.json is as follows:

    {
  "database": {
    "rules": "database.rules.json"
  },
  "hosting": {
    "public": "build/unbundled",
    "rewrites": [
      {
        "source": "!/__/**",
        "destination": "/index.html"
      },
      {
        "source": "**/!(*.js|*.html|*.css|*.json|*.svg|*.png|*.jpg|*.jpeg)",
        "destination": "/index.html"
      }
    ]
  }
}

How do I get it to display extra.html when i navigate to https://firebaseUrl/extra.html? Thanks.