Add tests for timers in background isolates in Dartium.

tests/html/async_periodictimer.dart

+library async_periodictimer;
+
+import 'dart:async';
+import 'package:unittest/unittest.dart';
+
+main(message, replyTo) {
+  var command = message.first;
+  expect(command, 'START');
+  int counter = 0;
+  new Timer.periodic(const Duration(milliseconds: 10), (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: 30), () {
+        replyTo.send('DONE');
+      });
+      return;
+    }
+    assert(counter < 3);
+    counter++;
+  });
+}