Fix readLineSync to work in checked mode.

sdk/lib/io/stdio.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.
 
 part of dart.io;
 
 const int _STDIO_HANDLE_TYPE_TERMINAL = 0;
 const int _STDIO_HANDLE_TYPE_PIPE = 1;
 const int _STDIO_HANDLE_TYPE_FILE = 2;
 const int _STDIO_HANDLE_TYPE_SOCKET = 3;
@@ skipping 11 matching lines @@
                                         bool cancelOnError}) {
     return _stream.listen(
         onData,
         onError: onError,
         onDone: onDone,
         cancelOnError: cancelOnError);
   }
 }
 
 
-class _StdinEventSink {
+class _StdinEventSink implements EventSink<String> {
   Function add;
   Function addError;
   Function close;
   _StdinEventSink(this.add, this.addError, this.close);
 }
 
 /**
  * [Stdin] allows both synchronous and asynchronous reads from the standard
  * input stream.
  *
@@ skipping 170 matching lines @@
   }
   return StdioType.OTHER;
 }
 
 
 class _StdIOUtils {
   external static IOSink _getStdioOutputStream(int fd);
   external static Stdin _getStdioInputStream();
   external static int _socketType(nativeSocket);
 }