Rework service extensions to be safe

sdk/lib/developer/extension.dart

Index: sdk/lib/developer/extension.dart
diff --git a/sdk/lib/developer/extension.dart b/sdk/lib/developer/extension.dart
index f23b5ad5630b266f35c0ef1576cedf6595374ff8..24820f423139f8d42fbcc1db55e6ca14014f03de 100644
--- a/sdk/lib/developer/extension.dart
+++ b/sdk/lib/developer/extension.dart
@@ -102,17 +102,17 @@ void registerExtension(String method, ServiceExtensionHandler handler) {
   _extensions[method] = handler;
 }
 
-bool _scheduleExtension(String method,
-                        List<String> parameterKeys,
-                        List<String> parameterValues,
-                        SendPort replyPort,
-                        Object id) {
+Function _scheduleExtension(String method,
+                            List<String> parameterKeys,
+                            List<String> parameterValues,
+                            SendPort replyPort,
+                            Object id) {
   ServiceExtensionHandler handler = _extensions[method];
   if (handler == null) {
-    return false;
+    return null;
   }
-  // Defer execution of handler until next event loop.
-  Timer.run(() {
+  // This closure will be executed at the next turn of the event loop.
+  return () {
     var parameters = {};
     for (var i = 0; i < parameterKeys.length; i++) {
       parameters[parameterKeys[i]] = parameterValues[i];
@@ -148,8 +148,7 @@ bool _scheduleExtension(String method,
       }
       _postResponse(replyPort, id, response);
     });
-  });
-  return true;
+  };
 }
 
 _postResponse(SendPort replyPort,