Polymer.json for differential serving

polymer.json

{
  ...
  "builds": [
    {
      "name": "modern-es6",
      "browserCapabilities": ["es2015"],
      "preset": "es6-unbundled",
      "basePath": true
    }, {
      "name": "fallback-es5",
      "preset": "es5-bundled",
      "basePath": true
    }
  ]
}