Fix strong mode errors in dart:io.

sdk/lib/io/stdio.dart

Index: sdk/lib/io/stdio.dart
diff --git a/sdk/lib/io/stdio.dart b/sdk/lib/io/stdio.dart
index feea4fffc08249eb5d100e1b81c210fec024f138..8fb196dbaae0e55c63dd85117146077cb63c90dd 100644
--- a/sdk/lib/io/stdio.dart
+++ b/sdk/lib/io/stdio.dart
@@ -328,13 +328,19 @@ StdioType stdioType(object) {
     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) {
+      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;
@@ -350,6 +356,6 @@ StdioType stdioType(object) {
 class _StdIOUtils {
   external static _getStdioOutputStream(int fd);
   external static Stdin _getStdioInputStream();
-  external static int _socketType(nativeSocket);
+  external static int _socketType(Socket socket);
   external static _getStdioHandleType(int fd);
 }