Fix subscription handling in stream decoder

tests/standalone/io/process_stdin_transform_unsubscribe_script.dart

Index: tests/standalone/io/process_stdin_transform_unsubscribe_script.dart
diff --git a/tests/standalone/io/process_stdin_transform_unsubscribe_script.dart b/tests/standalone/io/process_stdin_transform_unsubscribe_script.dart
new file mode 100644
index 0000000000000000000000000000000000000000..071112488a9d03915796316a3d51a058b0a830cf
--- /dev/null
+++ b/tests/standalone/io/process_stdin_transform_unsubscribe_script.dart
@@ -0,0 +1,18 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// Utility script to echo stdin to stdout or stderr or both.
+
+import "dart:io";
+
+main() {
+  var subscription;
+  subscription = stdin
+      .transform(new StringDecoder())
+      .transform(new LineTransformer())
+      .listen((String line) {
+          // Unsubscribe after the first line.
+          subscription.cancel();
+        });
+}