Fix problem when running new test in checked mode.

runtime/observatory/tests/service/capture_stdio_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--compile-all --error_on_bad_type --error_on_bad_override
 
 import 'dart:async';
 import 'dart:developer';
 import 'dart:io';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 14 matching lines @@
   hasStoppedAtBreakpoint,
 
   (Isolate isolate) async {
     Completer completer = new Completer();
     var stdoutSub;
     stdoutSub = await isolate.vm.listenEventStream(
       VM.kStdoutStream,
       (ServiceEvent event) {
         expect(event.kind, equals('WriteEvent'));
         expect(event.bytesAsString, equals('stdout'));
-        cancelFutureSubscription(stdoutSub).then((_) {
+        stdoutSub.cancel().then((_) {
           completer.complete();
         });
       });
     await isolate.resume();
     await completer.future;
   },
 
   hasStoppedAtBreakpoint,
 
   (Isolate isolate) async {
     Completer completer = new Completer();
     var stdoutSub;
     int eventNumber = 1;
     stdoutSub = await isolate.vm.listenEventStream(
       VM.kStdoutStream,
       (ServiceEvent event) {
         expect(event.kind, equals('WriteEvent'));
         if (eventNumber == 1) {
           expect(event.bytesAsString, equals('print'));
         } else if (eventNumber == 2) {
           expect(event.bytesAsString, equals('\n'));
-          cancelFutureSubscription(stdoutSub).then((_) {
+          stdoutSub.cancel().then((_) {
             completer.complete();
           });
         } else {
           expect(true, false);
         }
         eventNumber++;
       });
     await isolate.resume();
     await completer.future;
   },
 
   hasStoppedAtBreakpoint,
 
   (Isolate isolate) async {
     Completer completer = new Completer();
     var stderrSub;
     stderrSub = await isolate.vm.listenEventStream(
       VM.kStderrStream,
       (ServiceEvent event) {
         expect(event.kind, equals('WriteEvent'));
         expect(event.bytesAsString, equals('stderr'));
-        cancelFutureSubscription(stderrSub).then((_) {
+        stderrSub.cancel().then((_) {
           completer.complete();
         });
       });
     await isolate.resume();
     await completer.future;
   },
 ];
 
   
 main(args) => runIsolateTests(args, tests, testeeConcurrent: test);