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

runtime/bin/socket_macos.cc

Index: runtime/bin/socket_macos.cc
diff --git a/runtime/bin/socket_macos.cc b/runtime/bin/socket_macos.cc
index d4f205e85915020aaa21414b6de0fa0e8acdd7b4..d1988693c1dca01460b3bd15622503d068ff25bc 100644
--- a/runtime/bin/socket_macos.cc
+++ b/runtime/bin/socket_macos.cc
@@ -45,6 +45,15 @@ 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 Mac OS.
   return true;