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 +48,26 @@ function run( args, options ) {
 		const filePath = path.resolve( process.cwd(), files[ i ] );
 		delete require.cache[ filePath ];
 
+		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 simply use that unconditionally on
+			// newer versions regardless of the given file path.
+			// 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
+			//
+			// Node.js 12.0.0 has node_module_version=72
+			// https://nodejs.org/en/download/releases/
+			//
+			// https://github.com/qunitjs/qunit/issues/1465
+			if ( !nodeVint || nodeVint >= 72 ) {
+				await import( filePath );
+			} else {
+				require( filePath );
+			}
 		} 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

@@ -135,6 +135,17 @@ 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, "" );
+			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;