str.slice is not a function when using experimentalCodeSplitting and the input is an object

[Couto]

According to Rollup's documentation is possible to pass an object of named entry points to the input property if we enable the experimentalCodeSplitting.

However, this seems to break whenever the commonjs plugin is being used:

TypeError: str.slice is not a function
    at startsWith (node_modules/rollup-plugin-commonjs/dist/rollup-plugin-commonjs.cjs.js:633:13)
    at resolveId (node_modules/rollup-plugin-commonjs/dist/rollup-plugin-commonjs.cjs.js:675:23)
    at node_modules/rollup-plugin-commonjs/dist/rollup-plugin-commonjs.cjs.js:763:49
    at Array.map (<anonymous>)
    at Object.options (node_modules/rollup-plugin-commonjs/dist/rollup-plugin-commonjs.cjs.js:763:18)
    at applyOptionHook (node_modules/rollup/dist/rollup.js:22009:23)
    at Array.reduce (<anonymous>)
    at getInputOptions$1 (node_modules/rollup/dist/rollup.js:22032:41)
    at Object.rollup (node_modules/rollup/dist/rollup.js:22093:30)
    at build (node_modules/rollup/bin/rollup:1151:19)

error Command failed with exit code 1.

Theres a small demo of this problem in the following gist to demonstrate the problem.

[Couto]

Closing this issue in favour of #329