Fix a Socket edge-case where the socket is closed by peer right after accept, where we havn't set a…

runtime/bin/socket_patch.dart

Index: runtime/bin/socket_patch.dart
diff --git a/runtime/bin/socket_patch.dart b/runtime/bin/socket_patch.dart
index 0bcf75fd0a2899163b5f00e3fb5bf40447e70c3a..4d9225daa8170ddce139fd878df64d4757de8992 100644
--- a/runtime/bin/socket_patch.dart
+++ b/runtime/bin/socket_patch.dart
@@ -490,7 +490,9 @@ class _NativeSocket extends NativeFieldWrapperClass1 {
         if (i == ERROR_EVENT) {
           reportError(nativeGetError(), "");
         } else if (!isClosed) {
-          handler();
+          // If the connection is closed right after it's accepted, there's a
+          // chance the close-handler is not set.
+          if (handler != null) handler();
         }
       }
     }