Add a test for timers in child isolates.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 018b8aa93a10c2ead94f064616a433dc50934513..06214ba0e495c06983329e4ede952ecc08942a29 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -32785,29 +32785,45 @@ class _PureIsolateTimer implements Timer {
   final ReceivePort _port = new ReceivePort();
   SendPort _sendPort; // Effectively final.
 
+  static SendPort _SEND_PORT;
+
   _PureIsolateTimer(int milliSeconds, callback, repeating) {
     _sendPort = _port.toSendPort();
     _port.receive((msg, replyTo) {
       assert(msg == _TIMER_PING);
-      assert(replyTo == _HELPER_ISOLATE_PORT);
       callback(this);
       if (!repeating) _cancel();
     });
-    _HELPER_ISOLATE_PORT.then((port) {
-      port.send([_NEW_TIMER, milliSeconds, repeating], _sendPort);
-    });
+
+    _send([_NEW_TIMER, milliSeconds, repeating]);
   }
 
   void cancel() {
     _cancel();
-    _HELPER_ISOLATE_PORT.then((port) {
-      port.send([_CANCEL_TIMER], _sendPort);
-    });
+    _send([_CANCEL_TIMER]);
   }
 
   void _cancel() {
     _port.close();
   }
+
+  // Tricky part.
+  // Once _HELPER_ISOLATE_PORT gets resolved, it will still delay in .then
+  // and to delay Timer.run is used. However, Timer.run will try to register
+  // another Timer and here we got stuck: event cannot be posted as then
+  // callback is not executed because it's delayed with timer.
+  // Therefore once future is resolved, it's unsafe to call .then on it
+  // in Timer code.
+  _send(msg) {
+    if (_SEND_PORT != null) {
+      _SEND_PORT.send(msg, _sendPort);
+    } else {
+      _HELPER_ISOLATE_PORT.then((port) {
+        _SEND_PORT = port;
+        _SEND_PORT.send(msg, _sendPort);
+      });
+    }
+  }
 }
 
 get _pureIsolateTimerFactoryClosure =>