Wait for exit-code thread to terminate before terminating main.

runtime/bin/process.h

Index: runtime/bin/process.h
diff --git a/runtime/bin/process.h b/runtime/bin/process.h
index 117c5c008477bab29aea4d3c8a75514b37186777..fdf23243c89958824969c87ce3762c07385c00b8 100644
--- a/runtime/bin/process.h
+++ b/runtime/bin/process.h
@@ -31,6 +31,10 @@ class Process {
   // Indicate that the process with the given pid has exited.
   static void Exit(intptr_t id);
 
+  // Terminate the exit code handler thread. Does not return before
+  // the thread has terminated.
+  static void TerminateExitCodeHandler();
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Process);
 };