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

runtime/vm/jit_optimizer.cc

Index: runtime/vm/jit_optimizer.cc
diff --git a/runtime/vm/jit_optimizer.cc b/runtime/vm/jit_optimizer.cc
index 172f5d241645708f30d10b589c1c408f759a8971..f05813d0b7628bc0412b0683b41ea6c5a37dad15 100644
--- a/runtime/vm/jit_optimizer.cc
+++ b/runtime/vm/jit_optimizer.cc
@@ -2917,9 +2917,10 @@ void JitOptimizer::VisitStoreInstanceField(
     // usage count of at least 1/kGetterSetterRatio of the getter usage count.
     // This is to avoid unboxing fields where the setter is never or rarely
     // executed.
-    const Field& field = Field::ZoneHandle(Z, instr->field().Original());
+    const Field& field = instr->field();
     const String& field_name = String::Handle(Z, field.name());
-    const Class& owner = Class::Handle(Z, field.Owner());
+    const Class& owner =
+        Class::Handle(Z, Field::Handle(Z, field.Original()).Owner());
     const Function& getter =
         Function::Handle(Z, owner.LookupGetterFunction(field_name));
     const Function& setter =
@@ -2954,6 +2955,7 @@ void JitOptimizer::VisitStoreInstanceField(
           OS::Print("  getter usage count: %" Pd "\n", getter.usage_counter());
         }
       }
+      ASSERT(field.IsOriginal());
       field.set_is_unboxing_candidate(false);
       field.DeoptimizeDependentCode();
     } else {