Always mark stdout and stderr as blocking on exit.

runtime/bin/process.cc

Index: runtime/bin/process.cc
diff --git a/runtime/bin/process.cc b/runtime/bin/process.cc
index f3ed86a682cbd6f1c5ac93c2ac50727ad964e178..89c8028075e15c53c4c6e10fd1d13d5e2361066f 100644
--- a/runtime/bin/process.cc
+++ b/runtime/bin/process.cc
@@ -4,6 +4,7 @@
 
 #include "bin/dartutils.h"
 #include "bin/io_buffer.h"
+#include "bin/platform.h"
 #include "bin/process.h"
 #include "bin/socket.h"
 
@@ -211,6 +212,8 @@ void FUNCTION_NAME(Process_Exit)(Dart_NativeArguments args) {
   int64_t status = 0;
   // Ignore result if passing invalid argument and just exit 0.
   DartUtils::GetInt64Value(Dart_GetNativeArgument(args, 0), &status);
+  // Be sure to do platform-specific cleanups.
+  Platform::Cleanup();
   exit(static_cast<int>(status));
 }