CLI: Add native support for ESM .mjs files on Node 12+

* Rename test/es2017 and update ESLint config to allow for import/export
  syntax to be used. We don't rely on ESLint to know what is supported
  in Node.js, our test matrix in CI covers that already.

  We should probably re-think the way these tests are organized,
  which I've filed #1511 for.

* Update eslint config for src/cli to es2020. The dynamic `import()`
  statement was only spe'ced in es2020. This is first implemented
  (without experimental flag) in Node 12. We need to support Node 10.
  However it looks like Node 10 already tolerated this (maybe it sees
  it as a normal function, or maybe it was already recognised by the
  V8 parser it shipped with), so the try-catch suffices there.
  Again, the CI test matrix verifies for us that stuff works fine
  on older versions.

Ref #1465.

.eslintrc.json

@@ -52,7 +52,7 @@
             ],
             "parserOptions": {
                 "sourceType": "script",
-                "ecmaVersion": 2018
+                "ecmaVersion": 2020
             },
             "env": {
                 "node": true

Gruntfile.js

@@ -178,8 +178,8 @@ module.exports = function( grunt ) {
 				"test/module-only",
 				"test/module-skip",
 				"test/module-todo",
-				"test/es2017/async-functions",
-				"test/es2017/throws"
+				"test/es2018/async-functions",
+				"test/es2018/throws"
 			]
 		},
 		"watch-repeatable": {

src/cli/run.js

@@ -12,7 +12,7 @@ const changedPendingPurge = [];
 
 let QUnit;
 
-function run( args, options ) {
+async function run( args, options ) {
 
 	// Default to non-zero exit code to avoid false positives
 	process.exitCode = 1;
@@ -48,8 +48,42 @@ function run( args, options ) {
 		const filePath = path.resolve( process.cwd(), files[ i ] );
 		delete require.cache[ filePath ];
 
+		// Node.js 12.0.0 has node_module_version=72
+		// https://nodejs.org/en/download/releases/
+		const nodeVint = process.config.variables.node_module_version;
+
 		try {
-			require( filePath );
+
+			// QUnit supports passing ESM files to the 'qunit' command when used on
+			// Node.js 12 or later. The dynamic import() keyword supports both CommonJS files
+			// (.js, .cjs) and ESM files (.mjs), so we could simply use that unconditionally on
+			// newer Node versions, regardless of the given file path.
+			//
+			// But:
+			// - Node.js 12 emits a confusing "ExperimentalWarning" when using import(),
+			//   even if just to load a non-ESM file. So we should try to avoid it on non-ESM.
+			// - This Node.js feature is still considered experimental so to avoid unexpected
+			//   breakage we should continue using require(). Consider flipping once stable and/or
+			//   as part of QUnit 3.0.
+			// - Plugins and CLI bootstrap scripts may be hooking into require.extensions to modify
+			//   or transform code as it gets loaded. For compatibility with that, we should
+			//   support that until at least QUnit 3.0.
+			// - File extensions are not sufficient to differentiate between CJS and ESM.
+			//   Use of ".mjs" is optional, as a package may configure Node to default to ESM
+			//   and optionally use ".cjs" for CJS files.
+			//
+			// https://nodejs.org/docs/v12.7.0/api/modules.html#modules_addenda_the_mjs_extension
+			// https://nodejs.org/docs/v12.7.0/api/esm.html#esm_code_import_code_expressions
+			// https://github.com/qunitjs/qunit/issues/1465
+			try {
+				require( filePath );
+			} catch ( e ) {
+				if ( e.code === "ERR_REQUIRE_ESM" && ( !nodeVint || nodeVint >= 72 ) ) {
+					await import( filePath );
+				} else {
+					throw e;
+				}
+			}
 		} catch ( e ) {
 
 			// eslint-disable-next-line no-loop-func

src/cli/utils.js

@@ -27,6 +27,8 @@ function getFilesFromArgs( args ) {
 
 	// Default to files in the test directory
 	if ( !patterns.length ) {
+
+		// TODO: In QUnit 3.0, change the default to {js,mjs}
 		patterns.push( "test/**/*.js" );
 	}
 
@@ -45,6 +47,8 @@ function getFilesFromArgs( args ) {
 			files.add( pattern );
 		} else {
 			if ( stat && stat.isDirectory() ) {
+
+				// TODO: In QUnit 3.0, change the default to {js,mjs}
 				pattern = `${pattern}/**/*.js`;
 			}
 			const results = glob( pattern, {

test/cli/fixtures/expected/tap-outputs.js

@@ -203,6 +203,15 @@ not ok 2 Example > bad
 # fail 1
 `,
 
+	"qunit ../../es2018/esm.mjs":
+`TAP version 13
+ok 1 ESM test suite > sum\\(\\)
+1..1
+# pass 1
+# skip 0
+# todo 0
+# fail 0`,
+
 	"qunit timeout":
 `TAP version 13
 not ok 1 timeout > first

test/cli/main.js

@@ -103,7 +103,14 @@ QUnit.module( "CLI Main", function() {
 			await execute( command );
 		} catch ( e ) {
 			assert.equal( e.code, 1 );
-			assert.equal( e.stderr, "" );
+
+			// Node 12 enabled ESM by default, without experimental flag,
+			// but left the warning in stderr. The warning was removed in Node 14.
+			// Don't bother checking stderr
+			if ( semver.gte( process.versions.node, "14.0.0" ) ) {
+				assert.equal( e.stderr, "" );
+			}
+
 			const re = new RegExp( expectedOutput[ command ] );
 			assert.equal( re.test( e.stdout ), true );
 		}
@@ -135,6 +142,21 @@ QUnit.module( "CLI Main", function() {
 		}
 	} );
 
+	if ( semver.gte( process.versions.node, "12.0.0" ) ) {
+		QUnit.test( "run ESM test suite with import statement", async function( assert ) {
+			const command = "qunit ../../es2018/esm.mjs";
+			const execution = await execute( command );
+
+			assert.equal( execution.code, 0 );
+			assert.equal( execution.stderr, "" );
+			const re = new RegExp( expectedOutput[ command ] );
+			assert.equal( re.test( execution.stdout ), true );
+			if ( !re.test( execution.stdout ) ) {
+				assert.equal( execution.stdout, expectedOutput[ command ] );
+			}
+		} );
+	}
+
 	// https://nodejs.org/dist/v12.12.0/docs/api/cli.html#cli_enable_source_maps
 	if ( semver.gte( process.versions.node, "14.0.0" ) ) {
 

test/es2018/.eslintrc.json

@@ -0,0 +1,6 @@
+{
+		"parserOptions": {
+				"ecmaVersion": 2018,
+				"sourceType": "module"
+		}
+}

test/es2018/esm.mjs

@@ -0,0 +1,9 @@
+import sum from "./sum.mjs";
+
+QUnit.module( "ESM test suite", () => {
+
+	QUnit.test( "sum()", assert => {
+		assert.equal( 5, sum( 2, 3 ) );
+	} );
+
+} );

test/es2018/sum.mjs

@@ -0,0 +1,5 @@
+function sum( a, b ) {
+	return a + b;
+}
+
+export default sum;