Regression (8.1.2): mocha/mocha.js is no longer bundle compatible (uses dynamic require)

[AviVahl]

Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

Starting with v8.1.2, mocha/mocha.js contains node-specific code (dynamic require call) and is no longer bundle compatible

Steps to Reproduce

mkdir mocha-bundle-test
cd mocha-bundle-test
npm init -y
npm i -D webpack webpack-cli mocha
mkdir src
echo "import mocha from 'mocha/mocha.js'; console.log(mocha);" > src/index.js
npx webpack --mode development

Expected behavior: [What you expect to happen]

No errors or warnings when trying to import and bundle mocha/mocha.js.
Same as 8.1.1.

Actual behavior: [What actually happens]

WARNING in ./node_modules/mocha/mocha.js 25044:26-47
Critical dependency: the request of a dependency is an expression
 @ ./src/index.js

Reproduces how often: [What percentage of the time does it reproduce?]
Consistently. 100% of the time.

Versions

npx envinfo

System:
    OS: Linux 5.7 Fedora 32 (Workstation Edition) 32 (Workstation Edition)
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
    Memory: 17.04 GB / 31.21 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 14.8.0 - ~/.nvm/versions/node/v14.8.0/bin/node
    Yarn: 1.22.4 - ~/.nvm/versions/node/v14.8.0/bin/yarn
    npm: 6.14.8 - ~/.nvm/versions/node/v14.8.0/bin/npm
  Managers:
    pip3: 19.3.1 - /usr/bin/pip3
    Yum: 4.2.23 - /usr/bin/yum
  Utilities:
    Make: 4.2.1 - /usr/bin/make
    GCC: 10.2.1 - /usr/bin/gcc
    Git: 2.26.2 - /usr/bin/git
    FFmpeg: 4.2.4 - /usr/bin/ffmpeg
  Servers:
    Apache: 2.4.43 - /usr/sbin/apachectl
  IDEs:
    Nano: 4.9.3 - /usr/bin/nano
    VSCode: 1.48.2 - /usr/bin/code
  Languages:
    Bash: 5.0.17 - /usr/bin/bash
    Perl: 5.30.3 - /usr/bin/perl
    Python: 3.8.5 - /usr/bin/python
    Python3: 3.8.5 - /usr/bin/python3
  Databases:
    SQLite: 3.33.0 - /usr/bin/sqlite3
  Browsers:
    Chrome: 85.0.4183.83
    Firefox: 79.0

Additional Information

This issue breaks mocha-loader and mocha-pup.

[Munter]

Looks like Mocha.unloadFile and Mocha.prototype.unloadFiles should be removed from the browser bundle:

Here are the relevant pieces of code from the bundle. Seems webpack is breaking on the require.resolve() call, which should never be used in a browser context anyway:

	  Mocha.unloadFile = function (file) {
	    delete require.cache[require.resolve(file)];
	  };
	  /**
	   * Unloads `files` from Node's `require` cache.
	   *
	   * @description
	   * This allows required files to be "freshly" reloaded, providing the ability
	   * to reuse a Mocha instance programmatically.
	   * Note: does not clear ESM module files from the cache
	   *
	   * <strong>Intended for consumers &mdash; not used internally</strong>
	   *
	   * @public
	   * @see {@link Mocha#run}
	   * @returns {Mocha} this
	   * @chainable
	   */


	  Mocha.prototype.unloadFiles = function () {
	    if (this._state === mochaStates.DISPOSED) {
	      throw createMochaInstanceAlreadyDisposedError('Mocha instance is already disposed, it cannot be used again.', this._cleanReferencesAfterRun, this);
	    }

	    this.files.forEach(function (file) {
	      Mocha.unloadFile(file);
	    });
	    this._state = mochaStates.INIT;
	    return this;
	  };

[boneskull]

Fix for this is in #4419