NodeJS run_ API method should return native JavaScript Promise

Currently the `run_` method, exposed through the `dart.sass.js` file,
returns a `Future`. The `Future` object is not accessible due to the
Dart compilation process and `Future` not being a thing in the
JavaScript/NodeJS platform.

In order to provide a better interface to the Sass API for
JavaScript/NodeJS consumers, a native `Promise` is now returned.

Fixes bazelbuild/rules_sass#96.

lib/src/node.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 import 'dart:convert';
+import 'dart:js';
 import 'dart:js_util';
 import 'dart:typed_data';
 
@@ -21,6 +22,7 @@ import 'importer/node.dart';
 import 'node/error.dart';
 import 'node/exports.dart';
 import 'node/function.dart';
+import 'node/promise.dart';
 import 'node/render_context.dart';
 import 'node/render_context_options.dart';
 import 'node/render_options.dart';
@@ -39,8 +41,8 @@ import 'visitor/serialize.dart';
 /// export that runs the normal `main()`, which is called from `package/sass.js`
 /// to run the executable when installed from npm.
 void main() {
-  exports.run_ = allowInterop(
-      (Object args) => executable.main(List.from(args as List<Object>)));
+  exports.run_ = allowInterop((Object args) =>
+      futureToPromise(executable.main(List.from(args as List<Object>))));
   exports.render = allowInterop(_render);
   exports.renderSync = allowInterop(_renderSync);
   exports.info =

lib/src/node/promise.dart

@@ -0,0 +1,19 @@
+// Copyright 2019 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'dart:js';
+
+import 'package:js/js.dart';
+
+/// Describes the native JavaScript `Promise` global.
+@JS()
+class Promise<T> {
+  external Promise(void executor(void resolve(T result), Function reject));
+}
+
+/// Converts a Future to a JavaScript Promise.
+Promise<T> futureToPromise<T>(Future<T> future) {
+  return Promise<T>(allowInterop((resolve, reject) =>
+      future.then(resolve, onError: reject)));
+}

test/node_api/api.dart

@@ -15,6 +15,7 @@ export 'package:sass/src/node/render_context.dart';
 export 'package:sass/src/node/render_options.dart';
 export 'package:sass/src/node/render_result.dart';
 import 'package:sass/src/node/fiber.dart';
+import 'package:sass/src/node/promise.dart';
 import 'package:sass/src/node/render_options.dart';
 import 'package:sass/src/node/render_result.dart';
 
@@ -50,6 +51,7 @@ external FiberClass _requireFiber(String path);
 
 @JS()
 class Sass {
+  external Promise<void> run_(Object args);
   external RenderResult renderSync(RenderOptions args);
   external void render(RenderOptions args,
       void callback(RenderError error, RenderResult result));

test/node_api/exports_test.dart

@@ -0,0 +1,37 @@
+// Copyright 2018 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+@TestOn('node')
+@Tags(['node'])
+
+import 'package:js/js.dart';
+import 'package:js/js_util.dart';
+import 'package:sass/src/node/promise.dart';
+import 'package:test/test.dart';
+
+import '../ensure_npm_package.dart';
+import 'api.dart';
+import 'utils.dart';
+
+/// Describes a JavaScript object.
+///
+/// Object's always have a `constructor` property in JavaScript.
+@JS()
+class ObjectWithConstructor {
+  Object constructor;
+}
+
+void main() {
+  setUpAll(ensureNpmPackage);
+  useSandbox();
+
+  group('run_ method', () {
+    test('returns a JavaScript native Promise', () async {
+      var result = sass.run_([]) as ObjectWithConstructor;
+      expect(result, isA<Promise<void>>());
+      expect(result.constructor.toString(), contains('[native code]'));
+      await promiseToFuture(result);
+    });
+  });
+}