[dart:io] Adds a finalizer to _NativeSocket to avoid socket leaks

runtime/bin/socket_linux.cc

Index: runtime/bin/socket_linux.cc
diff --git a/runtime/bin/socket_linux.cc b/runtime/bin/socket_linux.cc
index 23427a740e465c8eb3c667d77229074334a99112..108186ffc834ddc7d4885f1cd6c8fda399af6199 100644
--- a/runtime/bin/socket_linux.cc
+++ b/runtime/bin/socket_linux.cc
@@ -46,6 +46,15 @@ bool Socket::FormatNumericAddress(const RawAddr& addr, char* address, int len) {
 }
 
 
+Socket::Socket(intptr_t fd)
+    : ReferenceCounted(), fd_(fd), port_(ILLEGAL_PORT) {}
+
+
+void Socket::SetClosedFd() {
+  fd_ = kClosedFd;
+}
+
+
 bool Socket::Initialize() {
   // Nothing to do on Linux.
   return true;