VM: Fix assertion failure from background compiler.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index b184f6cce2a3bbfb3ceff21de569e12dabc6a94f..8ed08737a0cdfbcf2035f1acc675afbb7ab959c8 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2016,7 +2016,7 @@ void Isolate::set_registered_service_extension_handlers(
 
 void Isolate::AddDeoptimizingBoxedField(const Field& field) {
   ASSERT(Compiler::IsBackgroundCompilation());
-  ASSERT(field.IsOriginal());
+  ASSERT(!field.IsOriginal());
   // The enclosed code allocates objects and can potentially trigger a GC,
   // ensure that we account for safepoints when grabbing the lock.
   SafepointMutexLocker ml(field_list_mutex_);
@@ -2025,7 +2025,7 @@ void Isolate::AddDeoptimizingBoxedField(const Field& field) {
   }
   const GrowableObjectArray& array =
       GrowableObjectArray::Handle(boxed_field_list_);
-  array.Add(field, Heap::kOld);
+  array.Add(Field::Handle(field.Original()), Heap::kOld);
 }