Background compilation work:

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index fa212ea6d81bbe78eed8b2a409954ab24e1e9608..dc2abcc09e0f902ab0c7e6f1e94833d8042dae34 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -4393,6 +4393,7 @@ class Code : public Object {
 
   void Enable() const {
     if (!IsDisabled()) return;
+    ASSERT(Thread::Current()->IsMutatorThread());
     ASSERT(instructions() != active_instructions());
     set_active_instructions(instructions());
   }
@@ -4447,6 +4448,7 @@ class Code : public Object {
   }
 
   void set_active_instructions(RawInstructions* instructions) const {
+    ASSERT(Thread::Current()->IsMutatorThread() || !is_alive());
     // RawInstructions are never allocated in New space and hence a
     // store buffer update is not needed here.
     StorePointer(&raw_ptr()->active_instructions_, instructions);
@@ -4456,6 +4458,7 @@ class Code : public Object {
   }
 
   void set_instructions(RawInstructions* instructions) const {
+    ASSERT(Thread::Current()->IsMutatorThread() || !is_alive());
     StorePointer(&raw_ptr()->instructions_, instructions);
   }