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

tests/lib_strong/html/html_mock_test.dart

Index: tests/lib_strong/html/html_mock_test.dart
diff --git a/tests/lib_strong/html/html_mock_test.dart b/tests/lib_strong/html/html_mock_test.dart
index bdf88600e4cf9b80978e022bba80a78b63da8649..b470495181cb5079587208341a119f84cc34f7d6 100644
--- a/tests/lib_strong/html/html_mock_test.dart
+++ b/tests/lib_strong/html/html_mock_test.dart
@@ -16,13 +16,19 @@ 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";
@@ -65,4 +71,11 @@ main() {
     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');
+  });
 }