core-hackery.js

let internalCoreReference;

export function __initCore__(core) {
  if (internalCoreReference) {
    throw new Error('Core already set');
  }
  internalCoreReference = core;
}

export function getCore() {
  if (!internalCoreReference) {
    throw new Error('Core not set');
  }
  return internalCoreReference;
}

plugin.js

class Plugin {
  setup(core) {
    require('./core-hackery').__initCore__(core);
    core.applications.register({
      id: 'foo',
      async mount(domElement) {
        const bootstrapApp = await import('./application');
        return bootstrapApp(domElement);
      }
    });
  }
}

and consumer:

// application.js
import { getCore } from './core-hackery';
const core = getCore();

where do i put the core-hackery.js file? In public or server code?

It seems that it is on the server code. I'm confused how I can use it inside hacks code?

This example is all client-side new platform plugins. It's not directly applicable to a legacy plugin since that doesn't have a controlled lifecycle.