Closes https://github.com/dart-lang/sdk/issues/28291

tests/lib_strong/html/js_extend_class_test.dart

+// Copyright (c) 2017, 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.
+
+@JS()
+library js_extend_class_test;
+
+import 'dart:html';
+
+import 'package:js/js.dart';
+import 'package:js/js_util.dart' as js_util;
+import 'package:expect/minitest.dart';
+
+@JS('Date')
+class JSDate {
+  external get jsField;
+  external get jsMethod;
+}
+
+@JS('Date.prototype.jsField')
+external set datePrototypeJSField(v);
+
+@JS('Date.prototype.jsMethod')
+external set datePrototypeJSMethod(v);
+
+// Extending a JS class with a Dart class is only supported by DDC for now.
+// We extend the Date class instead of a user defined JS class to avoid the
+// hassle of ensuring the JS class exists before we use it.
+class DartJsDate extends JSDate {
+  get dartField => 100;
+  int dartMethod(x) {
+    return x * 2;
+  }
+}
+
+main() {
+  // Monkey-patch the JS Date class.
+  datePrototypeJSField = 42;
+  datePrototypeJSMethod = allowInterop((x) => x * 10);
+
+  group('extend js class', () {
+    test('js class members', () {
+      var bar = new DartJsDate();
+      expect(bar.jsField, equals(42));
+      expect(bar.jsMethod(5), equals(50));
+
+      expect(bar.dartField, equals(100));
+      expect(bar.dartMethod(4), equals(8));
+    });
+
+    test('dart subclass members', () {
+      var bar = new DartJsDate();
+      expect(bar.dartField, equals(100));
+      expect(bar.dartMethod(4), equals(8));
+    });
+  });
+}