Revert "Removing setImmediate and hiding WindowTimers interface"

tests/html/microtask_test.dart

+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library microtask_;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:async';
+import 'dart:html';
+
+main() {
+  useHtmlConfiguration();
+
+  test('setImmediate', () {
+    var timeoutCalled = false;
+    var rafCalled = false;
+    var immediateCalled = false;
+
+    Timer.run(expectAsync0(() {
+      timeoutCalled = true;
+      expect(immediateCalled, true);
+    }));
+
+
+    window.requestAnimationFrame((_) {
+      rafCalled = true;
+    });
+
+    window.setImmediate(expectAsync0(() {
+      expect(timeoutCalled, false);
+      expect(rafCalled, false);
+      immediateCalled = true;
+    }));
+    expect(immediateCalled, false);
+  });
+}