Add RunnerSuite.{on,is}Debugging properties.

lib/src/runner/browser/iframe_listener.dart

Index: lib/src/runner/browser/iframe_listener.dart
diff --git a/lib/src/runner/browser/iframe_listener.dart b/lib/src/runner/browser/iframe_listener.dart
index 18a938d8a77f0000462a1e3ae3667560ce3cea4f..2a658cf3033a6d017dc836fa82986045a3070b11 100644
--- a/lib/src/runner/browser/iframe_listener.dart
+++ b/lib/src/runner/browser/iframe_listener.dart
@@ -40,6 +40,11 @@ class IframeListener {
   static Future start(Function getMain()) async {
     var channel = _postMessageChannel();
 
+    // Send periodic pings to the test runner so it can know when the suite is
+    // paused for debugging.
+    new Timer.periodic(new Duration(seconds: 1),
+        (_) => channel.sink.add({"type": "ping"}));
+
     var main;
     try {
       main = getMain();