Where should I insert '@babel/polyfill' into?: main.js or each modules

Question

I'm trying to implement web application using Babel 7, @babel/preset-env and @babel/polyfill.

// main.js
import a from 'a';

a.doSomething()

// a.js
export default class {
    doSomething() {
        // assume that here is some code that needs polyfills
    }
}

If I should insert import '@babel/polyfill' to support old browsers, where should it be, either in main.js or a.js, or both of them?