Add an integration test to verify that server I/O is asynchronous.

pkg/analysis_server/test/timing/timing_framework.dart

Index: pkg/analysis_server/test/timing/timing_framework.dart
diff --git a/pkg/analysis_server/test/timing/timing_framework.dart b/pkg/analysis_server/test/timing/timing_framework.dart
index cbaf1e61c1ff3a19a4701a5c8cca6a7807d379cf..cf10ee4e006fca4fd619a3cae903cfe6953e79a6 100644
--- a/pkg/analysis_server/test/timing/timing_framework.dart
+++ b/pkg/analysis_server/test/timing/timing_framework.dart
@@ -89,7 +89,8 @@ abstract class TimingTest extends IntegrationTestMixin {
     onServerConnected.listen((_) {
       serverConnected.complete();
     });
-    return server.start(dispatchNotification).then((params) {
+    return server.start().then((params) {
+      server.listenToOutput(dispatchNotification);
       server.exitCode.then((_) {
         skipShutdown = true;
       });