Enable explicit .m.js intent for ESM

doc/api/esm.md

@@ -19,11 +19,13 @@ and implementation is ready. Error messages are still being polished.
 The `--experimental-modules` flag can be used to enable features for loading
 ESM modules.
 
-Once this has been set, files ending with `.mjs` will be able to be loaded
+Once this has been set, files ending with `.mjs` or `.m.js` will be able to be loaded
 as ES Modules.
 
 ```sh
 node --experimental-modules my-app.mjs
+
+node --experimental-modules other-app.m.js
 ```
 
 ## Features
@@ -41,6 +43,7 @@ points into ESM graphs at run time.
 | Feature | Reason |
 | --- | --- |
 | `require('./foo.mjs')` | ES Modules have differing resolution and timing, use language standard `import()` |
+| `require('./foo.m.js')` | same as above, use language standard `import()` |
 | `import()` | pending newer V8 release used in Node.js |
 | `import.meta` | pending V8 implementation |
 | Loader Hooks | pending Node.js EP creation/consensus |

lib/internal/loader/ModuleRequest.js

@@ -114,7 +114,8 @@ exports.resolve = (specifier, parentURL) => {
     case '.node':
       return { url: `${url}`, format: 'addon' };
     case '.js':
-      return { url: `${url}`, format: 'cjs' };
+      const format = url.pathname.slice(-5) === '.m.js' ? 'esm' : 'cjs';
+      return { url: `${url}`, format };
     default:
       throw new errors.Error('ERR_UNKNOWN_FILE_EXTENSION',
                              internalURLModule.getPathFromURL(url));

lib/module.js

@@ -611,6 +611,9 @@ Module.prototype._compile = function(content, filename) {
 
 // Native extension for .js
 Module._extensions['.js'] = function(module, filename) {
+  if (experimentalModules && filename.slice(-5) === '.m.js') {
+    return Module._extensions['.mjs'](module, filename);
+  }
   var content = fs.readFileSync(filename, 'utf8');
   module._compile(internalModule.stripBOM(content), filename);
 };

test/es-module/test-es-m-basic-imports.mjs

@@ -0,0 +1,5 @@
+// Flags: --experimental-modules
+import assert from 'assert';
+import ok from './test-esm-ok.m.js';
+
+assert(ok === true);

test/es-module/test-es-m-failing-require.js

@@ -0,0 +1,9 @@
+// Flags: --experimental-modules
+const assert = require('assert');
+
+try {
+  require('./test-esm-ok.m.js');
+  assert(false);
+} catch(error) {
+  assert(/es\s*m(?:odule)?|\.m\.?js/i.test(error.message));
+}

test/es-module/test-esm-ok.m.js

@@ -0,0 +1,5 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+const isJs = true;
+export default isJs;