Don't call accept the c socket, if we're currently closing.

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 a3cc62be9335d3a74f9623a6d71b8f18fac021a1..c24ba4df3a531f3b84ec142a5205352fa9daac19 100644
--- a/runtime/bin/socket_patch.dart
+++ b/runtime/bin/socket_patch.dart
@@ -426,6 +426,8 @@ class _NativeSocket extends NativeFieldWrapperClass1 {
   }
 
   _NativeSocket accept() {
+    // Don't issue accept if we're closing.
+    if (isClosing || isClosed) return null;
     var socket = new _NativeSocket.normal();
     if (nativeAccept(socket) != true) return null;
     socket.localPort = localPort;