Rename unsubscribe to cancel.

tests/standalone/io/test_extension_fail_test.dart

 // Copyright (c) 2012, 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.
 //
 // Dart test program for testing native extensions.
 
 import "dart:io";
 
 Future copyFileToDirectory(Path file, Path directory) {
   String src = file.toNativePath();
@@ skipping 36 matching lines @@
                       testDirectory).then((_) {
     Path extensionDartFile = scriptDirectory.append('test_extension.dart');
     return copyFileToDirectory(extensionDartFile, testDirectory);
   }).then((_) {
     Path testExtensionTesterFile =
         scriptDirectory.append('test_extension_fail_tester.dart');
     return copyFileToDirectory(testExtensionTesterFile, testDirectory);
   }).then((_) {
     Path script = testDirectory.append('test_extension_fail_tester.dart');
     return Process.run(options.executable, [script.toNativePath()]);
-  }).subscribe(onValue: (ProcessResult result) {
+  }).listen((ProcessResult result) {
     print("ERR: ${result.stderr}\n\n");
     print("OUT: ${result.stdout}\n\n");
     Expect.equals(255, result.exitCode);
     Expect.isTrue(result.stderr.contains("Unhandled exception:\nball\n"));
     tempDirectory.deleteSync(recursive: true);
   }, onError: (_) {
     tempDirectory.deleteSync(recursive: true);
   });
 }