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) {

[Florian Schneider, 2016/05/09 09:05:15]

Why do the is-test again here?

[floitsch, 2016/05/09 12:48:01]

Where is the other test?

Basically, `nativeSocket` was guaranteed to be a `_nativeSocket` from a
`_Socket`. The `_nativeSocket` was extracted on the non-patch side, which looked
wrong, since that side shouldn't know about the `_Socket` class (which is
declared in the patch-side).
I changed the signature to take a `Socket` class instead, and check here, if it
really is a `_Socket` instead.

[Florian Schneider, 2016/05/09 13:39:43]

Sorry, missed the _.

+      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";