Re-enable support for default exports (#2009)

See #2008

CHANGELOG.md

@@ -2,6 +2,13 @@
 
 ### JavaScript API
 
+* Re-enable support for `import sass from 'sass'` when loading the package from
+  an ESM module in Node.js. However, this syntax is now deprecated; ESM users
+  should use `import * as sass from 'sass'` instead.
+
+  On the browser and other ESM-only platforms, only `import * as sass from
+  'sass'` is supported.
+
 * Properly export the legacy API values `TRUE`, `FALSE`, `NULL`, and `types` from
   the ECMAScript module API.
 

tool/grind.dart

@@ -105,6 +105,10 @@ void main(List<String> args) {
   };
 
   pkg.addAllTasks();
+
+  afterTask("pkg-npm-dev", _addDefaultExport);
+  afterTask("pkg-npm-release", _addDefaultExport);
+
   grind(args);
 }
 
@@ -247,3 +251,37 @@ dart run protoc_plugin "\$@"
             Platform.environment["PATH"]!
       }));
 }
+
+/// After building the NPM package, add default exports to
+/// `build/npm/sass.node.mjs`.
+///
+/// See sass/dart-sass#2008.
+void _addDefaultExport() {
+  var buffer = StringBuffer();
+  buffer.writeln(File("build/npm/sass.node.mjs").readAsStringSync());
+
+  buffer.writeln("""
+let printedDefaultExportDeprecation = false;
+function defaultExportDeprecation() {
+  if (printedDefaultExportDeprecation) return;
+  printedDefaultExportDeprecation = true;
+  console.error(
+      "`import sass from 'sass'` is deprecated.\\n" +
+      "Please use `import * as sass from 'sass'` instead.");
+}
+""");
+
+  buffer.writeln("export default {");
+  for (var export in pkg.jsEsmExports.value!) {
+    buffer.write("""
+  get $export() {
+    defaultExportDeprecation();
+    return _cliPkgExports.$export;
+  },
+""");
+  }
+
+  buffer.writeln("};");
+
+  File("build/npm/sass.node.mjs").writeAsStringSync(buffer.toString());
+}

tool/grind/utils.dart

@@ -2,13 +2,17 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:cli_pkg/cli_pkg.dart' as pkg;
 import 'package:grinder/grinder.dart';
 import 'package:path/path.dart' as p;
 
+// Work around the lack of google/grinder.dart#402.
+import 'package:grinder/src/singleton.dart';
+
 /// Options for [run] that tell Git to commit using SassBot's name and email.
 final sassBotEnvironment = RunOptions(environment: {
   "GIT_AUTHOR_NAME": pkg.botName.value,
@@ -75,3 +79,21 @@ String cloneOrCheckout(String url, String ref, {String? name}) {
 
   return path;
 }
+
+/// Registers [callback] to run after the task named [taskName].
+///
+/// This must be called after the base [taskName] is registered.
+void afterTask(String taskName, FutureOr<void> callback()) {
+  // This takes advantage of the fact that Grinder's task list is mutable to
+  // override the existing task with our new one.
+  var index = grinder.tasks.indexWhere((task) => task.name == taskName);
+  if (index == -1) fail("There is no task named $taskName.");
+
+  var oldTask = grinder.tasks[index];
+  grinder.tasks[index] = GrinderTask(taskName,
+      description: oldTask.description,
+      depends: oldTask.depends, taskFunction: (TaskArgs args) async {
+    await oldTask.execute(context, args);
+    await callback();
+  });
+}