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 834f3e1734d2f8d95fffdcfc0f06a7d182bcdf06..9708b71286512de650230b4c1ae581af3cf41986 100644
--- a/lib/src/runner/browser/iframe_listener.dart
+++ b/lib/src/runner/browser/iframe_listener.dart
@@ -39,6 +39,11 @@ 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();