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

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index 7e2cecb8aa2e354ce43ee852f76acaa03252464e..06e190958b0b5985b9a11d1609775166f41af727 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -1256,6 +1256,7 @@ bool RunMainIsolate(const char* script_name, CommandLineOptions* dart_options) {
       Log::PrintErr("VM cleanup failed: %s\n", error);
       free(error);
     }
+    Process::ClearAllSignalHandlers();
     EventHandler::Stop();
     Platform::Exit((exit_code != 0) ? exit_code : kErrorExitCode);
   }
@@ -1643,6 +1644,7 @@ void main(int argc, char** argv) {
     Log::PrintErr("VM cleanup failed: %s\n", error);
     free(error);
   }
+  Process::ClearAllSignalHandlers();
   EventHandler::Stop();
 
   delete app_snapshot;