Dart: Fixes rtt benchmark

fixes #776

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1991843002 .

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 6a3e760..8d1f1e0 100644
--- a/benchmarks/mojo_rtt_benchmark/lib/main.dart
+++ b/benchmarks/mojo_rtt_benchmark/lib/main.dart
@@ -47,7 +47,7 @@
     // 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);