Add all dart:html tests from the sdk to test/codegen.

test/codegen/lib/html/audioelement_test.dart

+library AudioElementTest;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+
+main() {
+  useHtmlConfiguration();
+
+  test('constructorTest1', () {
+      var audio = new AudioElement();
+      expect(audio, isNotNull);
+      expect(audio is AudioElement, isTrue);
+    });
+
+  test('constructorTest2', () {
+      var audio = new AudioElement('IntentionallyMissingFileURL');
+      expect(audio, isNotNull);
+      expect(audio is AudioElement, isTrue);
+      expect(audio.src, contains('IntentionallyMissingFileURL'));
+    });
+
+  test('canPlayTypeTest', () {
+      var audio = new AudioElement();
+      var canPlay = audio.canPlayType("audio/mp4");
+      expect(canPlay, isNotNull);
+      expect(canPlay is String, isTrue);
+    });
+}