could not create sourcemap

[yunnysunny]

I modified the polymer.json in icon-toggle-finished folder:

{
  "npm": true,
  "moduleResolution": "node",
  "entrypoint": "demo/index.html",
  "builds": [       
    { 
      "name": "es5prod",
      "preset": "es5-bundled",
      "bundle": {
        "sourcemaps": true,
        "inlineCss": true,
        "inlineScripts": false
      }
    }
  ]
}

And then run polymer build, but it didn't create sourcemap.