Check in codegen test expectations.

test/codegen/expect/lib/html/mouse_event_test.js

Index: test/codegen/expect/lib/html/mouse_event_test.js
diff --git a/test/codegen/expect/lib/html/mouse_event_test.js b/test/codegen/expect/lib/html/mouse_event_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..a25dd65a7a0c9178718d5d6a178bf0768d4f0513
--- /dev/null
+++ b/test/codegen/expect/lib/html/mouse_event_test.js
@@ -0,0 +1,28 @@
+dart_library.library('lib/html/mouse_event_test', null, /* Imports */[
+  'dart_sdk',
+  'unittest'
+], function load__mouse_event_test(exports, dart_sdk, unittest) {
+  'use strict';
+  const core = dart_sdk.core;
+  const html = dart_sdk.html;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const html_config = unittest.html_config;
+  const unittest$ = unittest.unittest;
+  const src__matcher__expect = unittest.src__matcher__expect;
+  const src__matcher__core_matchers = unittest.src__matcher__core_matchers;
+  const mouse_event_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  mouse_event_test.main = function() {
+    html_config.useHtmlConfiguration();
+    unittest$.test('relatedTarget', dart.fn(() => {
+      let event = html.MouseEvent.new('mouseout');
+      src__matcher__expect.expect(event[dartx.relatedTarget], src__matcher__core_matchers.isNull);
+      event = html.MouseEvent.new('mouseout', {relatedTarget: html.document[dartx.body]});
+      src__matcher__expect.expect(event[dartx.relatedTarget], html.document[dartx.body]);
+    }, VoidTodynamic()));
+  };
+  dart.fn(mouse_event_test.main, VoidTodynamic());
+  // Exports:
+  exports.mouse_event_test = mouse_event_test;
+});