Add tests for timers in background isolates in Dartium.

tests/html/async_cancellingisolate.dart

+library async_cancellingisolate;
+
+import 'dart:async';
+import 'package:unittest/unittest.dart';
+
+main(message, replyTo) {
+  var command = message.first;
+  expect(command, 'START');
+  var shot = false;
+  var oneshot;
+  var periodic;
+  periodic = new Timer.periodic(const Duration(milliseconds: 10), (timer) {
+    expect(shot, isFalse);
+    shot = true;
+    expect(timer, same(periodic));
+    periodic.cancel();
+    oneshot.cancel();
+    // Wait some more time to be sure callbacks won't be invoked any
+    // more.
+    new Timer(const Duration(milliseconds: 50), () {
+      replyTo.send('DONE');
+    });
+  });
+  // We launch the oneshot timer after the periodic timer. Otherwise a
+  // (very long) context switch could make this test flaky: assume the
+  // oneshot timer is created first and then there is a 30ms context switch.
+  // when the periodic timer is scheduled it would execute after the oneshot.
+  oneshot = new Timer(const Duration(milliseconds: 30), () {
+    fail('Should never be invoked');
+  });
+}