VM: Fix race between background compiler and guarded cid update.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index c5175145f9c47f76577c91c2ab47bd3b531459e4..0f6491243d6b2290c0fca9d0211b0b81cefddd0d 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3125,10 +3125,19 @@ class Field : public Object {
   // Return class id that any non-null value read from this field is guaranteed
   // to have or kDynamicCid if such class id is not known.
   // Stores to this field must update this information hence the name.
-  intptr_t guarded_cid() const { return raw_ptr()->guarded_cid_; }
+  intptr_t guarded_cid() const {
+#if defined(DEGUG)
+    Thread* thread = Thread::Current();
+    ASSERT(!IsOriginal() || thread->IsMutator() || thread->IsAtSafepoint());
+#endif
+    return raw_ptr()->guarded_cid_;
+  }
 
   void set_guarded_cid(intptr_t cid) const {
-    ASSERT(Thread::Current()->IsMutatorThread());
+#if defined(DEGUG)
+    Thread* thread = Thread::Current();
+    ASSERT(!IsOriginal() || thread->IsMutator() || thread->IsAtSafepoint());
+#endif
     StoreNonPointer(&raw_ptr()->guarded_cid_, cid);
   }
   static intptr_t guarded_cid_offset() {
@@ -3213,6 +3222,10 @@ class Field : public Object {
   // Deoptimize all dependent code objects.
   void DeoptimizeDependentCode() const;
 
+  // Used by background compiler to check consistency of field copy with its
+  // original.
+  bool IsConsistentWith(const Field& field) const;
+
   bool IsUninitialized() const;
 
   void EvaluateInitializer() const;