Dart: Fixes rtt benchmark

benchmarks/mojo_rtt_benchmark/lib/main.dart

Index: benchmarks/mojo_rtt_benchmark/lib/main.dart
diff --git a/benchmarks/mojo_rtt_benchmark/lib/main.dart b/benchmarks/mojo_rtt_benchmark/lib/main.dart
index 6a3e760242ad3e3c2a17047de239d33986eb1383..8d1f1e00116dc444639f97ccfaa4c1bdc7a814d6 100644
--- a/benchmarks/mojo_rtt_benchmark/lib/main.dart
+++ b/benchmarks/mojo_rtt_benchmark/lib/main.dart
@@ -47,7 +47,7 @@ class EchoTracingApp extends Application {
     // Setup the connection to the echo app.
     for (int i = 0; i < _numClients; i++) {
       var newProxy = new EchoProxy.unbound();
-      newProxy.errorFuture.then((e) {
+      newProxy.ctrl.errorFuture.then((e) {
         _errorHandler(e);
       });
       connectToService(echoUrl, newProxy);