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

runtime/bin/socket_fuchsia.cc

Index: runtime/bin/socket_fuchsia.cc
diff --git a/runtime/bin/socket_fuchsia.cc b/runtime/bin/socket_fuchsia.cc
index 5ea7f5207a7c8965fdff6c1a10df2605da065d36..0ecae8e4fd638a23e4d9e277ddf1024b1b57743a 100644
--- a/runtime/bin/socket_fuchsia.cc
+++ b/runtime/bin/socket_fuchsia.cc
@@ -72,6 +72,15 @@ 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 Fuchsia.
   return true;