Fix checked mode error on socket IOSink

tests/standalone/io/stdout_stderr_test.dart

+// 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.
+
+import "dart:async";
+import "dart:io";
+
+callIOSink(IOSink sink) {
+  // Call all methods on IOSink.
+  sink.encoding = Encoding.ASCII;
+  Expect.equals(Encoding.ASCII, sink.encoding);
+  sink.write("Hello\n");
+  sink.writeln("Hello");
+  sink.writeAll(["H", "e", "l", "lo\n"]);
+  sink.writeCharCode(72);
+  sink.writeBytes([101, 108, 108, 111, 10]);
+
+  var controller = new StreamController();
+  sink.writeStream(controller.stream);
+  controller.add([72, 101, 108]);
+  controller.add([108, 111, 10]);
+  controller.close();
+
+  controller = new StreamController();
+  controller.stream.pipe(sink);
+  controller.add([72, 101, 108]);
+  controller.add([108, 111, 10]);
+  controller.close();
+}
+
+main() {
+  callIOSink(stdout);
+  stdout.done.then((_) {
+    callIOSink(stderr);
+    stderr.done.then((_) {
+      stdout.close();
+      stderr.close();
+    });
+  });
+}