Fix background compilation issues when creating snapshots.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 6732c1b374ac3fa3803d194779a788a3ed014a97..287cbdca14242cbe353792ed7a02897539f0b122 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -196,9 +196,11 @@ class Heap {
   // Associate an id with an object (used when serializing an object).
   // A non-existant id is equal to 0.
   void SetObjectId(RawObject* raw_obj, intptr_t object_id) {
+    ASSERT(Thread::Current()->IsMutatorThread());
     SetWeakEntry(raw_obj, kObjectIds, object_id);
   }
   intptr_t GetObjectId(RawObject* raw_obj) const {
+    ASSERT(Thread::Current()->IsMutatorThread());
     return GetWeakEntry(raw_obj, kObjectIds);
   }
   int64_t ObjectIdCount() const;