Fix strong mode errors in dart:io.

runtime/bin/stdio_patch.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.
 
 patch class _StdIOUtils {
   static Stdin _getStdioInputStream() {
     switch (_getStdioHandleType(0)) {
       case _STDIO_HANDLE_TYPE_TERMINAL:
       case _STDIO_HANDLE_TYPE_PIPE:
       case _STDIO_HANDLE_TYPE_SOCKET:
@@ skipping 11 matching lines @@
       case _STDIO_HANDLE_TYPE_TERMINAL:
       case _STDIO_HANDLE_TYPE_PIPE:
       case _STDIO_HANDLE_TYPE_SOCKET:
       case _STDIO_HANDLE_TYPE_FILE:
         return new Stdout._(new IOSink(new _StdConsumer(fd)), fd);
       default:
         throw new FileSystemException("Unsupported stdin type");
     }
   }
 
-  static int _socketType(nativeSocket) {
-    var result = _getSocketType(nativeSocket);
-    if (result is OSError) {
-      throw new FileSystemException("Error retrieving socket type", "", result);
+  static int _socketType(Socket socket) {
+    if (socket is _Socket) {
+      var nativeSocket = socket._nativeSocket;
+      var result = _getSocketType(nativeSocket);
+      if (result is OSError) {
+        throw new FileSystemException(
+            "Error retrieving socket type", "", result);
+      }
+      return result;
     }
-    return result;
+    return null;
   }
 
   static _getStdioHandleType(int fd) native "File_GetStdioHandleType";
 }
 
 patch class Stdin {
   /* patch */ int readByteSync() native "Stdin_ReadByte";
 
   /* patch */ bool get echoMode => _echoMode;
   /* patch */ void set echoMode(bool enabled) { _echoMode = enabled; }
@@ skipping 27 matching lines @@
     }
     return size;
   }
 
   static _getTerminalSize(int fd) native "Stdout_GetTerminalSize";
 }
 
 
 _getStdioHandle(_NativeSocket socket, int num) native "Socket_GetStdioHandle";
 _getSocketType(_NativeSocket nativeSocket) native "Socket_GetType";