Use native wrapper fields to store socket ids.

runtime/bin/socket.h

Index: runtime/bin/socket.h
diff --git a/runtime/bin/socket.h b/runtime/bin/socket.h
index 5d68561a80cac9b58d479a514a7ea5906869ad46..82a7b890ea9cf0411ca4308b4ff1aee6d11b605c 100644
--- a/runtime/bin/socket.h
+++ b/runtime/bin/socket.h
@@ -36,7 +36,7 @@ class Socket {
   static int Write(intptr_t fd, const void* buffer, intptr_t num_bytes);
   static intptr_t CreateConnect(const char* host, const intptr_t port);
   static intptr_t GetPort(intptr_t fd);
-  static bool GetRemotePeer(intptr_t fd, char *host, intptr_t *port);
+  static bool GetRemotePeer(intptr_t fd, char* host, intptr_t* port);
   static void GetError(intptr_t fd, OSError* os_error);
   static intptr_t GetStdioHandle(int num);
 
@@ -46,6 +46,9 @@ class Socket {
 
   static Dart_Port GetServicePort();
 
+  static Dart_Handle SetSocketIdNativeField(Dart_Handle socket, intptr_t id);
+  static Dart_Handle GetSocketIdNativeField(Dart_Handle socket, intptr_t* id);
+
  private:
   static dart::Mutex mutex_;
   static int service_ports_size_;