Add bundler integration tests verifying EXPORT_ES6 output has no requ…

[vogel76]

Summary

Adds two integration tests to test_other.py that demonstrate how require() calls in EXPORT_ES6 output break bundlers. Both tests fail on main and are intended to pass after the require() elimination changes from #26384.

This addresses the review comment requesting concrete test cases showing bundler breakage.

Tests added

test_webpack_esm_output_clean

Compiles with -sEXPORT_ES6 and default environment (web+node), then builds with webpack.

Failure on main:

ERROR in node:module
Module build failed: UnhandledSchemeError: Reading from "node:module" is not handled by plugins (Unhandled scheme).
Webpack supports "data:" and "file:" URIs by default.
You may need an additional plugin to handle "node:" URIs.

 @ ./src/hello.mjs 83:34-55
 @ ./src/index.mjs 14:0-33 15:0-6

webpack 5.105.2 compiled with 1 error

Why it fails: Emscripten's EXPORT_ES6 output uses import { createRequire } from 'node:module' to polyfill require() for loading Node.js built-ins (fs, path, url). Webpack targeting web cannot resolve node: URI schemes — it hard-errors and produces no bundle at all. This breaks any webpack-based project (including Next.js, Nuxt, and Create React App) that tries to use emscripten's ESM output.

test_vite_esm_output_clean

Compiles with -sEXPORT_ES6 using default environment (web+node), then runs vite build and checks for externalization warnings.

Failure on main:

[plugin vite:resolve] Module "node:module" has been externalized for browser
compatibility, imported by "/src/out/test/hello.mjs".

AssertionError: Expected to NOT find 'externalized for browser compatibility'

Why it fails: Vite encounters import { createRequire } from 'node:module' in the ESM input. Since vite targets browsers, it can't bundle Node.js built-in modules. Instead of erroring, vite externalizes node:module — the build succeeds but the resulting bundle contains code that references modules unavailable in browsers. While the hello_world test case works by accident (the node code path is guarded by if (ENVIRONMENT_IS_NODE) which is false in browsers), any application with more complex node/web interop would silently break.

Rollup (not tested, for reference)

Rollup is the most lenient bundler — it warns about the unresolved dependency but still produces a bundle (exit code 0). The require() calls are passed through as-is into the output:

(!) Unresolved dependencies
https://rollupjs.org/troubleshooting/#warning-treating-module-as-external-dependency
node:module (imported by "hello.mjs")
created dist/bundle.mjs in 35ms
EXIT_CODE=0

The resulting bundle contains 4 require() calls. In Node.js this works because createRequire provides a working require function. In browsers the node code path is guarded by if (ENVIRONMENT_IS_NODE) so it's never reached — the bundle runs without error but carries dead node code. Because rollup doesn't error or even fail the build, it's a weaker test case and was not included in the test suite.

Bundler behavior summary

Bundler	Behavior on main	Exit code	Test
webpack	Hard error: UnhandledSchemeError on node:module	1	test_webpack_esm_output_clean
vite	Warning: externalizes node:module for browser compat	0	test_vite_esm_output_clean
rollup	Warning: unresolved dependency, passes require() through	0	(not tested)

Relationship to #26384

These tests validate the fix in #26384 which replaces require() with await import() in EXPORT_ES6 output. After cherry-picking this commit onto that branch, both tests should pass.