Fix strong mode errors in dart:io.

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 310 matching lines @@
     switch (_StdIOUtils._getStdioHandleType(object == stdout ? 1 : 2)) {
       case _STDIO_HANDLE_TYPE_TERMINAL: return StdioType.TERMINAL;
       case _STDIO_HANDLE_TYPE_PIPE: return StdioType.PIPE;
       case _STDIO_HANDLE_TYPE_FILE:  return StdioType.FILE;
     }
   }
   if (object is _FileStream) {
     return StdioType.FILE;
   }
   if (object is Socket) {
-    switch (_StdIOUtils._socketType(object._nativeSocket)) {
-      case _STDIO_HANDLE_TYPE_TERMINAL: return StdioType.TERMINAL;
-      case _STDIO_HANDLE_TYPE_PIPE: return StdioType.PIPE;
-      case _STDIO_HANDLE_TYPE_FILE:  return StdioType.FILE;
+    int socketType = _StdIOUtils._socketType(object);
+    if (socketType == null) return StdioType.OTHER;
+    switch (socketType) {
+      case _STDIO_HANDLE_TYPE_TERMINAL:
+        return StdioType.TERMINAL;
+      case _STDIO_HANDLE_TYPE_PIPE:
+        return StdioType.PIPE;
+      case _STDIO_HANDLE_TYPE_FILE:
+        return StdioType.FILE;
     }
   }
-  if (object is IOSink) {
+  if (object is _IOSinkImpl) {
     try {
       if (object._target is _FileStreamConsumer) {
         return StdioType.FILE;
       }
     } catch (e) {
       // Only the interface implemented, _sink not available.
     }
   }
   return StdioType.OTHER;
 }
 
 
 class _StdIOUtils {
   external static _getStdioOutputStream(int fd);
   external static Stdin _getStdioInputStream();
-  external static int _socketType(nativeSocket);
+  /// Returns the socket type or `null` if [socket] is not a builtin socket.
+  external static int _socketType(Socket socket);
   external static _getStdioHandleType(int fd);
 }