Fix strong mode errors in dart:io.

runtime/bin/stdio_patch.dart

Index: runtime/bin/stdio_patch.dart
diff --git a/runtime/bin/stdio_patch.dart b/runtime/bin/stdio_patch.dart
index 7e66009655ce1c76856d4b56dc08751de6c8cc29..c7e425866459812dd7b833ea9b4becda09c75b0d 100644
--- a/runtime/bin/stdio_patch.dart
+++ b/runtime/bin/stdio_patch.dart
@@ -29,12 +29,17 @@ patch class _StdIOUtils {
     }
   }
 
-  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";