Do not JSON encode the 'result' of a service rpc.

runtime/observatory/tests/service/gc_test.dart

Index: runtime/observatory/tests/service/gc_test.dart
diff --git a/runtime/observatory/tests/service/gc_test.dart b/runtime/observatory/tests/service/gc_test.dart
index 736ccc13a1c2d2b85d94048e65edaad3b2927149..49e653ea9921e1da527a5c33e55ff77e9c95d584 100644
--- a/runtime/observatory/tests/service/gc_test.dart
+++ b/runtime/observatory/tests/service/gc_test.dart
@@ -28,8 +28,11 @@ var tests = [
   // Expect at least this many GC events.
   int gcCountdown = 3;
   isolate.vm.events.stream.listen((ServiceEvent event) {
-    if (event.eventType == ServiceEvent.kGC && --gcCountdown == 0) {
-      completer.complete();
+    if (event.eventType == ServiceEvent.kGC) {
+      print('Received GC event');
+      if (--gcCountdown == 0) {
+        completer.complete();
+      }
     }
   });
   return completer.future;