fix #29233, final fields can be settable in a mock

tests/lib_strong/html/html_mock_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.
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:expect/minitest.dart';
 
 class Mock {
   noSuchMethod(Invocation i) => document;
 }
 
 @proxy
 class MockBodyElement extends Mock implements BodyElement {
   Node append(Node e) => e;
 }
 
+class _EventListeners {
+  Stream<Event> get onBlur => new Stream.fromIterable([]);
+}
+
 @proxy
-class MockHtmlDocument extends Mock implements HtmlDocument {
+class MockHtmlDocument extends Mock
+    with _EventListeners
+    implements HtmlDocument {
   BodyElement get body => new MockBodyElement();
 }
 
 @proxy
-class MockWindow extends Mock implements Window {
+class MockWindow extends Mock with _EventListeners implements Window {
   Stream<Event> get onBeforeUnload => new Stream.fromIterable([]);
 
   String name = "MOCK_NAME";
 }
 
 @proxy
 class MockLocation extends Mock implements Location {
   String href = "MOCK_HREF";
 }
 
@@ skipping 22 matching lines @@
     loc1.href = "RESET";
     expect(loc2.href, equals("RESET"));
     loc2.href = "RESET2";
     expect(loc3.href, equals("RESET2"));
   });
 
   test('method', () {
     HtmlDocument doc = new MockHtmlDocument();
     expect(doc.body.append(null), equals(null));
   });
+
+  test('mixin', () {
+    Window win = new MockWindow();
+    expect(win.onBlur is Stream, isTrue, reason: 'onBlur should be a stream');
+    HtmlDocument doc = new MockHtmlDocument();
+    expect(doc.onBlur is Stream, isTrue, reason: 'onBlur should be a stream');
+  });
 }