webpack bundle.js not found

According to your package.json you are using the new babel 6 release. However you don't have all the required dependencies for this new release. According to babel-loader you need to install babel-preset-es2015 too:

npm install babel-loader babel-core babel-preset-es2015 --save-dev

As you are also using React, you need the react preset to be installed too. So install both:

npm install --save-dev babel-preset-es2015 babel-preset-react

Create a file in your package folder called .babelrc to keep the babel configuration and enable both presets:

.babelrc

{
  "presets": ["es2015", "react"]
}

And then run the server again:

npm run dev

Then http://localhost:8090/assets/bundle.js should show the bundled module.

My diagnosis:

You cannot get the the bundle.js because your npm run dev throws some errors trying to apply the babel loader because it is not properly configured. Then, when you try to request the bundle.js file you get a 404 error because it has not been generated.