Bundle mixed JavaScript/TypeScript project with rollup

rollup.config.js

import typescript from 'rollup-plugin-typescript2';

export default {
  input: 'src/index.js',
  output: {
    file: 'dist/app.js',
    format: 'umd',
    name: 'app',
    sourcemap: true
  },
  plugins: [
    typescript({
      include: "src/**/*.{js,ts}"
    })
  ]
};

tsconfig.json

{
  "compilerOptions": {
    "target": "es6",
    "module": "es2015",
    "allowJs": true,
    "checkJs": true,
    "strict": true,
    "esModuleInterop": true
  },
  "include": [
    "src"
  ]
}