Removing outdated & flaky async_window_test

tests/html/async_window_test.dart

-library AsyncWindowTest;
-import '../../pkg/unittest/lib/unittest.dart';
-import '../../pkg/unittest/lib/html_config.dart';
-import 'dart:html';
-import 'dart:async';
-
-main() {
-  useHtmlConfiguration();
-  test('Timer', () {
-    new Timer(const Duration(milliseconds: 10), expectAsync0((){}));
-  });
-  test('Timer.periodic', () {
-    int counter = 0;
-    int id = null;
-    new Timer.periodic(const Duration(milliseconds: 10),
-        expectAsyncUntil1(
-        (timer) {
-          if (counter == 3) {
-            counter = 1024;
-            timer.cancel();
-            // Wait some more time to be sure callback won't be invoked any
-            // more.
-            new Timer(const Duration(milliseconds: 50), expectAsync0((){}));
-            return;
-          }
-          // As callback should have been cleared on 4th invocation, counter
-          // should never be greater than 3.
-          assert(counter < 3);
-          counter++;
-        },
-        () => counter == 3));
-  });
-}