Check in codegen test expectations.

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

+dart_library.library('lib/html/cross_domain_iframe_test', null, /* Imports */[
+  'dart_sdk',
+  'unittest'
+], function load__cross_domain_iframe_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 unittest$ = unittest.unittest;
+  const src__matcher__expect = unittest.src__matcher__expect;
+  const src__matcher__core_matchers = unittest.src__matcher__core_matchers;
+  const cross_domain_iframe_test = Object.create(null);
+  let MessageEventTodynamic = () => (MessageEventTodynamic = dart.constFn(dart.functionType(dart.dynamic, [html.MessageEvent])))();
+  let MessageEventTodynamic$ = () => (MessageEventTodynamic$ = dart.constFn(dart.definiteFunctionType(dart.dynamic, [html.MessageEvent])))();
+  let MessageEventTobool = () => (MessageEventTobool = dart.constFn(dart.definiteFunctionType(core.bool, [html.MessageEvent])))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  cross_domain_iframe_test.main = function() {
+    unittest$.test('cross_domain_iframe', dart.fn(() => {
+      let uri = core.Uri.parse(html.window[dartx.location][dartx.href]);
+      let crossOriginPort = core.int.parse(uri.queryParameters[dartx.get]('crossOriginPort'));
+      let crossOrigin = dart.str`${uri.scheme}://${uri.host}:${crossOriginPort}`;
+      let crossOriginUrl = dart.str`${crossOrigin}/root_dart/tests/html/cross_domain_iframe_script.html`;
+      let iframe = html.IFrameElement.new();
+      iframe[dartx.src] = crossOriginUrl;
+      html.document[dartx.body][dartx.append](iframe);
+      html.window[dartx.onMessage].where(dart.fn(event => event[dartx.origin] == crossOrigin, MessageEventTobool())).first.then(dart.dynamic)(MessageEventTodynamic()._check(unittest$.expectAsync(dart.fn(event => {
+        src__matcher__expect.expect(event[dartx.data], src__matcher__core_matchers.equals('foobar'));
+        src__matcher__expect.expect(event[dartx.source], src__matcher__core_matchers.isNotNull);
+      }, MessageEventTodynamic$()))));
+    }, VoidTodynamic()));
+  };
+  dart.fn(cross_domain_iframe_test.main, VoidTodynamic());
+  // Exports:
+  exports.cross_domain_iframe_test = cross_domain_iframe_test;
+});