esm: add experimental warning for --loader

Author: guybedford

lib/internal/process/esm_loader.js

@@ -7,6 +7,7 @@ const {
   ERR_INVALID_TYPE_FLAG,
   ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING,
 } = require('internal/errors').codes;
+const { emitExperimentalWarning } = require('internal/util');
 
 const type = require('internal/options').getOptionValue('--type');
 if (type && type !== 'commonjs' && type !== 'module')
@@ -48,6 +49,7 @@ exports.initializeLoader = function(cwd, userLoader) {
   let ESMLoader = new Loader();
   const loaderPromise = (async () => {
     if (userLoader) {
+      emitExperimentalWarning('--loader');
       const hooks = await ESMLoader.import(
         userLoader, pathToFileURL(`${cwd}/`).href);
       ESMLoader = new Loader();