Add Supported class to dart:html

tests/html/xhr_test.dart

Index: tests/html/xhr_test.dart
diff --git a/tests/html/xhr_test.dart b/tests/html/xhr_test.dart
index 0087b74a392b8f8f68c8bd5b3aae09583c92712e..4758a8a1038551642a34582e8276b701181c3f3d 100644
--- a/tests/html/xhr_test.dart
+++ b/tests/html/xhr_test.dart
@@ -143,7 +143,7 @@ main() {
     });
 
     test('XHR.request responseType arraybuffer', () {
-      if (ArrayBuffer.supported) {
+      if (Platform.supportsTypedData) {
         HttpRequest.request(url, responseType: 'arraybuffer').then(
           expectAsync1((xhr) {
             expect(xhr.status, equals(200));
@@ -166,7 +166,7 @@ main() {
 
   group('xhr_requestBlob', () {
     test('XHR.request responseType blob', () {
-      if (ArrayBuffer.supported) {
+      if (Platform.supportsTypedData) {
         return HttpRequest.request(url, responseType: 'blob').then(
           (xhr) {
             expect(xhr.status, equals(200));