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

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index 6fc4834dd1ef15ebe67c313fe65f67496976d213..e82d954f089869f983d91571f8b53bf425647c8c 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -496,8 +496,9 @@ static bool CheckDisablingFields(
   Zone* zone = thread->zone();
   Field& field = Field::Handle(zone, isolate->GetDisablingField());
   while (!field.IsNull()) {
+    ASSERT(field.IsOriginal());
     for (intptr_t i = 0; i < guarded_fields.length(); i++) {
-      if (guarded_fields.At(i)->raw() == field.raw()) {
+      if (guarded_fields.At(i)->Original() == field.raw()) {
         return true;
       }
     }
@@ -646,9 +647,10 @@ NOT_IN_PRODUCT(
 
       const ZoneGrowableArray<const Field*>& guarded_fields =
           *flow_graph->parsed_function().guarded_fields();
+      Field& field = Field::Handle();
       for (intptr_t i = 0; i < guarded_fields.length(); i++) {
-        const Field* field = guarded_fields[i];
-        field->RegisterDependentCode(code);
+        field = guarded_fields[i]->Original();
+        field.RegisterDependentCode(code);
       }
     }
   } else {  // not optimized.