Check in codegen test expectations.

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

+dart_library.library('lib/html/messageevent_test', null, /* Imports */[
+  'dart_sdk',
+  'unittest'
+], function load__messageevent_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 messageevent_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  messageevent_test.main = function() {
+    html_config.useHtmlConfiguration();
+    unittest$.test('new MessageEvent', dart.fn(() => {
+      let event = html.MessageEvent.new('type', {cancelable: true, data: 'data', origin: 'origin', lastEventId: 'lastEventId'});
+      src__matcher__expect.expect(event[dartx.type], src__matcher__core_matchers.equals('type'));
+      src__matcher__expect.expect(event[dartx.bubbles], src__matcher__core_matchers.isFalse);
+      src__matcher__expect.expect(event[dartx.cancelable], src__matcher__core_matchers.isTrue);
+      src__matcher__expect.expect(event[dartx.data], src__matcher__core_matchers.equals('data'));
+      src__matcher__expect.expect(event[dartx.origin], src__matcher__core_matchers.equals('origin'));
+      src__matcher__expect.expect(event[dartx.source], html.window);
+    }, VoidTodynamic()));
+  };
+  dart.fn(messageevent_test.main, VoidTodynamic());
+  // Exports:
+  exports.messageevent_test = messageevent_test;
+});