Make VM event streams look like real dart streams.

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 9d93b0a2295eaec6d359cf8da531928df38dc506..ac677dd543d9f275edeea72c4109a1981e09c48b 100644
--- a/runtime/observatory/tests/service/gc_test.dart
+++ b/runtime/observatory/tests/service/gc_test.dart
@@ -26,13 +26,16 @@ var tests = [
   Completer completer = new Completer();
   // Expect at least this many GC events.
   int gcCountdown = 3;
-  isolate.vm.events.stream.listen((ServiceEvent event) {
-    if (event.kind == ServiceEvent.kGC) {
+  isolate.vm.getGCEventStream().then((stream) {
+    var subscription;
+    subscription = stream.listen((ServiceEvent event) {
+      assert(event.kind == ServiceEvent.kGC);
       print('Received GC event');
       if (--gcCountdown == 0) {
+        subscription.cancel();
         completer.complete();
       }
-    }
+    });
   });
   return completer.future;
 },