Dart: Futures -> Callbacks.

examples/dart/echo_client/lib/main.dart

Index: examples/dart/echo_client/lib/main.dart
diff --git a/examples/dart/echo_client/lib/main.dart b/examples/dart/echo_client/lib/main.dart
index 02cffb04330b2d11d501be79b6b71e4df8011940..4f759231058f05f5d85acf152e8c9a46b679835b 100644
--- a/examples/dart/echo_client/lib/main.dart
+++ b/examples/dart/echo_client/lib/main.dart
@@ -24,9 +24,12 @@ class EchoClientApplication extends Application {
     final server = (arguments.length > 0) ? arguments[1] : "dart_echo_server";
     connectToService(url.replaceAll("dart_echo_client", server), _echo);
 
-    _echo.echoString("hello world").then((response) {
-      print("${response.value}");
-    }).whenComplete(_closeHandles);
+    var c = new Completer();
+    _echo.echoString("hello world", (String response) {
+      print("${response}");
+      c.complete(null);
+    });
+    c.future.whenComplete(_closeHandles);
   }
 
   Future _closeHandles() async {