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

runtime/bin/socket_android.cc

Index: runtime/bin/socket_android.cc
diff --git a/runtime/bin/socket_android.cc b/runtime/bin/socket_android.cc
index f9ed3489db825b4c1442ef7dc4d0be5ddb7a9f62..b2be713480a6fda28ebc29324383aab88d156e27 100644
--- a/runtime/bin/socket_android.cc
+++ b/runtime/bin/socket_android.cc
@@ -43,6 +43,15 @@ 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 Android.
   return true;