Add tests for timers in background isolates in Dartium.

tests/html/async_periodictimer.dart

Index: tests/html/async_periodictimer.dart
diff --git a/tests/html/async_periodictimer.dart b/tests/html/async_periodictimer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5284bad2782370a9504301c0c3585490ee1aab2a
--- /dev/null
+++ b/tests/html/async_periodictimer.dart
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+library async_periodictimer;
+
+import 'dart:async';
+import 'package:unittest/unittest.dart';
+
+main(message, replyTo) {
+  var command = message[0];

[terry, 2015/03/04 15:26:48]

ditto 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++;
+  });
+}