Fix for issue 21398 (only send "literal like" objects across isolates spawned using spawnURI

runtime/vm/benchmark_test.cc

Index: runtime/vm/benchmark_test.cc
===================================================================
--- runtime/vm/benchmark_test.cc	(revision 42788)
+++ runtime/vm/benchmark_test.cc	(working copy)
@@ -529,7 +529,7 @@
   timer.Start();
   for (intptr_t i = 0; i < kLoopCount; i++) {
     StackZone zone(isolate);
-    MessageWriter writer(&buffer, &message_allocator);
+    MessageWriter writer(&buffer, &message_allocator, true);
     writer.WriteMessage(null_object);
     intptr_t buffer_len = writer.BytesWritten();
 
@@ -552,7 +552,7 @@
   timer.Start();
   for (intptr_t i = 0; i < kLoopCount; i++) {
     StackZone zone(isolate);
-    MessageWriter writer(&buffer, &message_allocator);
+    MessageWriter writer(&buffer, &message_allocator, true);
     writer.WriteMessage(smi_object);
     intptr_t buffer_len = writer.BytesWritten();
 
@@ -577,7 +577,7 @@
   timer.Start();
   for (intptr_t i = 0; i < kLoopCount; i++) {
     StackZone zone(isolate);
-    MessageWriter writer(&buffer, &malloc_allocator);
+    MessageWriter writer(&buffer, &malloc_allocator, true);
     writer.WriteMessage(array_object);
     intptr_t buffer_len = writer.BytesWritten();