fix(generator): add core-js as direct dependency of generated projects

[haoqunjiang]

No description provided.

[cyrilf]

Hey @sodatea,

Could you explain me what is this change doing?
I'm a bit confused if I have to keep core-js as a dependency of my project or if I could move it to the dev-dependencies (or get rid of it all in all).

Thanks for your time! ✌️

[haoqunjiang]

@cyrilf
If you are using babel to transpile your code, babel may add some polyfill imports to the transpiled code for it to function correctly.
E.g.

const a = new Map()

will be transpiled into

import "core-js/modules/es6.map";
var a = new Map();

as map is not natively supported in IE9.

So technically speaking the core-js package is a dependency of your project.

In the past we utilized the package hoisting feature of npm and hide it behind @vue/cli-plugin-babel.
But after core-js 3 came out:

1. the internal directory structure of core-js changed a lot, we need to make sure users only get the correct version of core-js so as not to break their app
2. other packages in the project's dependency chain may have already depended on core-js@3, so we can't be sure if the root level core-js dependency is of the right version (as npm package hoisting is non-deterministic) without explicitly listing it in package.json.

To conclude:
you should keep it as a dependency.

[cyrilf]

Got it! 👍
Thanks a lot for this clear and explicit explanation. 👌

[zWingz]

Thanks for explanation @sodatea