[dart:js_interop] Add support for dynamically importing modules

Closes https://github.com/dart-lang/sdk/issues/52852

Allows importing of modules using the JS "import()" expression.
The result is a promise that resolves to a module, from which users
can then access exported members from.

CoreLibraryReviewExempt: Backend-specific library.
Change-Id: I4c1caae689be55d1dbb038448e3243bcb80ac8b5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/343683
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

sdk/lib/_internal/js_shared/lib/js_interop_patch.dart

@@ -448,3 +448,8 @@ extension JSAnyOperatorExtension on JSAny? {
   @pragma('dart2js:prefer-inline')
   bool get isTruthy => js_util.isTruthy(this);
 }
+
+@patch
+@pragma('dart2js:prefer-inline')
+JSPromise<JSObject> importModule(String moduleName) =>
+    foreign_helper.JS('', 'import(#)', moduleName);

sdk/lib/_internal/wasm/lib/js_interop_patch.dart

@@ -526,15 +526,20 @@ extension JSAnyOperatorExtension on JSAny? {
 
   @patch
   bool get not => _boxNonNullable<JSBoolean>(
-          js_helper.JS<WasmExternRef?>('(o) =>  !o', this.toExternRef))
+          js_helper.JS<WasmExternRef?>('(o) => !o', this.toExternRef))
       .toDart;
 
   @patch
   bool get isTruthy => _boxNonNullable<JSBoolean>(
-          js_helper.JS<WasmExternRef?>('(o) =>  !!o', this.toExternRef))
+          js_helper.JS<WasmExternRef?>('(o) => !!o', this.toExternRef))
       .toDart;
 }
 
+@patch
+JSPromise<JSObject> importModule(String moduleName) =>
+    _boxNonNullable<JSPromise<JSObject>>(js_helper.JS<WasmExternRef?>(
+        '(m) => import(m)', moduleName.toJS.toExternRef));
+
 @JS('Array')
 @staticInterop
 class _Array {

sdk/lib/js_interop/js_interop.dart

@@ -622,6 +622,8 @@ extension JSAnyOperatorExtension on JSAny? {
   external bool get isTruthy;
 }
 
+// Top-levels.
+
 /// Given a Dart object that is marked "exportable", creates a JS object that
 /// wraps the given Dart object. Look at the `@JSExport` annotation to determine
 /// what constitutes "exportable" for a Dart class. The object literal
@@ -667,3 +669,13 @@ external JSObject createJSInteropWrapper<T extends Object>(T dartObject);
 // `createStaticInteropMock`, we avoid introducing this method until later.
 // external T createJSInteropMock<T extends JSObject, U extends Object>(
 //     U dartMock, [JSObject? proto = null]);
+
+/// Call to dynamically import a JS module with the given [moduleName] using the
+/// JS `import()` syntax.
+///
+/// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import
+/// for more details.
+///
+/// Returns a [JSPromise] that resolves to a [JSObject] that's the module
+/// namespace object.
+external JSPromise<JSObject> importModule(String moduleName);

tests/lib/js/static_interop_test/import/import_test.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:js_interop';
+
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/minitest.dart';
+
+extension type Module(JSObject o) implements JSObject {
+  external String testModuleFunction();
+}
+
+void main() {
+  asyncTest(() async {
+    final module = Module(await importModule(
+            '/root_dart/tests/lib/js/static_interop_test/import/module.mjs')
+        .toDart);
+    expect(module.testModuleFunction(), 'success');
+  });
+}

tests/lib/js/static_interop_test/import/module.mjs

@@ -0,0 +1,3 @@
+export function testModuleFunction() {
+    return 'success';
+}

tests/lib/lib.status

@@ -33,6 +33,7 @@ js/export/static_interop_mock/proto_test: SkipByDesign # Uses dart:html.
 js/js_util/javascriptobject_extensions_test: SkipByDesign # Uses dart:html.
 js/static_interop_test/constants_test: SkipByDesign # Uses dart:html.
 js/static_interop_test/futurevaluetype_test: SkipByDesign # Uses dart:html.
+js/static_interop_test/import/import_test: SkipByDesign # TODO(srujzs): This test uses the file system to load a module. Since the test runner doesn't start an HTTP server for d8, I don't think this is supported.
 js/static_interop_test/supertype_transform_test: SkipByDesign # Uses dart:html.
 
 [ $runtime == dart_precompiled ]