[pkg:js] Handle null value in promise rejection

Closes https://github.com/dart-lang/sdk/issues/44602

Creates an exception to signal a `null`/`undefined` value when a
converted promise is rejected with `null`/`undefined`.

Change-Id: Ic7f14e23c6c1d51d6dbcc5831ffa8491418a4267
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/192046
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

sdk/lib/js_util/js_util.dart

@@ -149,6 +149,24 @@ dynamic callConstructor(Object constr, List<Object?>? arguments) {
   //     return _wrapToDart(jsObj);
 }
 
+/// Exception for when the promise is rejected with a `null` or `undefined`
+/// value.
+///
+/// This is public to allow users to catch when the promise is rejected with
+/// `null` or `undefined` versus some other value.
+class NullRejectionException implements Exception {
+  // Indicates whether the value is `undefined` or `null`.
+  final bool isUndefined;
+
+  NullRejectionException._(this.isUndefined);
+
+  @override
+  String toString() {
+    var value = this.isUndefined ? 'undefined' : 'null';
+    return 'Promise was rejected with a value of `$value`.';
+  }
+}
+
 /// Converts a JavaScript Promise to a Dart [Future].
 ///
 /// ```dart
@@ -163,7 +181,16 @@ Future<T> promiseToFuture<T>(Object jsPromise) {
   final completer = Completer<T>();
 
   final success = convertDartClosureToJS((r) => completer.complete(r), 1);
-  final error = convertDartClosureToJS((e) => completer.completeError(e), 1);
+  final error = convertDartClosureToJS((e) {
+    // Note that `completeError` expects a non-nullable error regardless of
+    // whether null-safety is enabled, so a `NullRejectionException` is always
+    // provided if the error is `null` or `undefined`.
+    if (e == null) {
+      return completer.completeError(
+          NullRejectionException._(JS('bool', '# === undefined', e)));
+    }
+    return completer.completeError(e);
+  }, 1);
 
   JS('', '#.then(#, #)', jsPromise, success, error);
   return completer.future;

tests/lib/js/js_util/promise_reject_null_test.dart

@@ -0,0 +1,64 @@
+@JS()
+library promise_reject_null_test;
+
+import 'package:js/js.dart';
+import 'package:js/js_util.dart' show promiseToFuture, NullRejectionException;
+
+import 'package:expect/minitest.dart';
+
+@JS()
+external void eval(String s);
+
+@JS('Promise.reject')
+external dynamic getRejectedPromise(v);
+
+@JS()
+external void reject(v);
+@JS()
+external dynamic getNewPromise();
+
+void main() async {
+  eval('''
+    self.getNewPromise = function () {
+      return new Promise(function (_, reject) {
+        self.reject = reject;
+      });
+    };
+  ''');
+
+  // Rejected promise with a `null` value should trigger a
+  // `NullRejectionException`.
+  await promiseToFuture(getRejectedPromise(null)).then((_) {
+    fail("Expected promise to reject and not fulfill.");
+  }).catchError((e) {
+    expect(e is NullRejectionException, true);
+    expect(e.isUndefined, false);
+  });
+
+  // Similar to the above, except we reject using JS interop.
+  var future = promiseToFuture(getNewPromise()).then((_) {
+    fail("Expected promise to reject and not fulfill.");
+  }).catchError((e) {
+    expect(e is NullRejectionException, true);
+    expect(e.isUndefined, false);
+  });
+
+  reject(null);
+
+  await future;
+
+  // It's also possible to reject with `undefined`. Make sure that the exception
+  // correctly flags that case.
+  future = promiseToFuture(getNewPromise()).then((_) {
+    fail("Expected promise to reject and not fulfill.");
+  }).catchError((e) {
+    expect(e is NullRejectionException, true);
+    expect(e.isUndefined, true);
+  });
+
+  eval('''
+    self.reject(undefined);
+  ''');
+
+  await future;
+}

tests/lib_2/js/js_util/promise_reject_null_test.dart

@@ -0,0 +1,64 @@
+@JS()
+library promise_reject_null_test;
+
+import 'package:js/js.dart';
+import 'package:js/js_util.dart' show promiseToFuture, NullRejectionException;
+
+import 'package:expect/minitest.dart';
+
+@JS()
+external void eval(String s);
+
+@JS('Promise.reject')
+external dynamic getRejectedPromise(v);
+
+@JS()
+external void reject(v);
+@JS()
+external dynamic getNewPromise();
+
+void main() async {
+  eval('''
+    self.getNewPromise = function () {
+      return new Promise(function (_, reject) {
+        self.reject = reject;
+      });
+    };
+  ''');
+
+  // Rejected promise with a `null` value should trigger a
+  // `NullRejectionException`.
+  await promiseToFuture(getRejectedPromise(null)).then((_) {
+    fail("Expected promise to reject and not fulfill.");
+  }).catchError((e) {
+    expect(e is NullRejectionException, true);
+    expect(e.isUndefined, false);
+  });
+
+  // Similar to the above, except we reject using JS interop.
+  var future = promiseToFuture(getNewPromise()).then((_) {
+    fail("Expected promise to reject and not fulfill.");
+  }).catchError((e) {
+    expect(e is NullRejectionException, true);
+    expect(e.isUndefined, false);
+  });
+
+  reject(null);
+
+  await future;
+
+  // It's also possible to reject with `undefined`. Make sure that the exception
+  // correctly flags that case.
+  future = promiseToFuture(getNewPromise()).then((_) {
+    fail("Expected promise to reject and not fulfill.");
+  }).catchError((e) {
+    expect(e is NullRejectionException, true);
+    expect(e.isUndefined, true);
+  });
+
+  eval('''
+    self.reject(undefined);
+  ''');
+
+  await future;
+}