Make stdout and stderr async.

tests/standalone/io/process_sync_script.dart

Index: tests/standalone/io/process_sync_script.dart
diff --git a/tests/standalone/io/process_sync_script.dart b/tests/standalone/io/process_sync_script.dart
index d8aab9c7b2858f6bc92203e377d2c7a6128d3ad4..5369124251414bd7ae0f4fee844dae37806d35ec 100644
--- a/tests/standalone/io/process_sync_script.dart
+++ b/tests/standalone/io/process_sync_script.dart
@@ -4,6 +4,7 @@
 //
 // Utility script to generate some output on stdout and stderr.
 
+import "dart:async";
 import "dart:math";
 import "dart:io";
 
@@ -20,5 +21,7 @@ main() {
     stdout.write(stdoutBlock);
     stderr.write(stderrBlock);
   }
-  exit(int.parse(options.arguments[3]));
+  Future.wait([stdout.close(), stderr.close()]).then((_) {
+    exit(int.parse(options.arguments[3]));
+  });
 }