lib: add warning on dynamic import es modules

PR-URL: https://github.com/nodejs/node/pull/30720
Reviewed-By: Guy Bedford <guybedford@gmail.com>

lib/internal/process/esm_loader.js

@@ -24,6 +24,12 @@ exports.initializeImportMetaObject = function(wrap, meta) {
 
 exports.importModuleDynamicallyCallback = async function(wrap, specifier) {
   assert(calledInitialize === true || !userLoader);
+  if (!calledInitialize) {
+    process.emitWarning(
+      'The ESM module loader is experimental.',
+      'ExperimentalWarning', undefined);
+    calledInitialize = true;
+  }
   const { callbackMap } = internalBinding('module_wrap');
   if (callbackMap.has(wrap)) {
     const { importModuleDynamically } = callbackMap.get(wrap);

test/es-module/test-esm-dynamic-import.js

@@ -42,6 +42,9 @@ function expectFsNamespace(result) {
 // For direct use of import expressions inside of CJS or ES modules, including
 // via eval, all kinds of specifiers should work without issue.
 (function testScriptOrModuleImport() {
+  common.expectWarning('ExperimentalWarning',
+                       'The ESM module loader is experimental.');
+
   // Importing another file, both direct & via eval
   // expectOkNamespace(import(relativePath));
   expectOkNamespace(eval(`import("${relativePath}")`));