Dart: Futures -> Callbacks.

benchmarks/mojo_rtt_benchmark/lib/main.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This Mojo app is a benchmark of Mojo/Dart IPC Round Trip Times.
 // It creates many proxies from a single isolate, and makes requests using those
 // proxies round-robin.
 // To run it, and the other RTT benchmarks:
 //
 // $ ./mojo/devtools/common/mojo_benchmark [--release] mojo/tools/data/rtt_benchmarks
@@ skipping 51 matching lines @@
     _warmup = true;
     new Timer(kWarmupDuration, () => _warmup = false);
   }
 
   _run(int idx) {
     if (idx == _numActiveClients) {
       idx = 0;
     }
     if (_doEcho) {
       if (_warmup) {
-        _echo(idx, "ping").then((r) {
+        _echo(idx, "ping", (String r) {
           new Timer(kDelay, () => _run(idx + 1));
         });
       } else {
-        _tracedEcho(idx, "ping").then((r) {
+        _tracedEcho(idx, "ping", (String r) {
           new Timer(kDelay, () => _run(idx + 1));
         });
       }
     }
   }
 
-  Future _tracedEcho(int idx, String s) {
+  void _tracedEcho(int idx, String s, void callback(String r)) {
     // This is not the correct way to use the Timeline API. In particular,
     // the start and end events below could occur on different threads.
     // This could mess up the interpretation of the trace by about:tracing.
     // We do this here as a hack to reduce overhead. The correct way to do
     // this would be to emit an async begin and end pair using TimelineTask.
     // Using the sync API means that we only record one event after the finish
     // time has been recorded.
     Timeline.startSync("ping");
-    return _echo(idx, s).then((r) {
+    _echo(idx, s, (String r) {
       Timeline.finishSync();
-      return r;
+      callback(r);
     });
   }
 
-  Future _echo(int idx, String s) {
-    return _echoProxies[idx].echoString(s);
+  void _echo(int idx, String s, void callback(String r)) {
+    _echoProxies[idx].echoString(s, callback);
   }
 
   _errorHandler(Object e) {
     _doEcho = false;
     return Future.wait(_echoProxies.map((p) => p.close())).then((_) {
       MojoHandle.reportLeakedHandles();
     });
   }
 }
 
 main(List args, Object handleToken) {
   MojoHandle appHandle = new MojoHandle(handleToken);
   new EchoTracingApp.fromHandle(appHandle);
 }