Use symbols when looking up fields in a class

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index c3954fd6a48721f1d2938c020a1b09f8933d67ea..66bc4c2e87cf381d8d18a39f09116257b00e5ccf 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -1493,10 +1493,10 @@ void Intrinsifier::Random_nextState(Assembler* assembler) {
       math_lib.LookupClassAllowPrivate(Symbols::_Random()));
   ASSERT(!random_class.IsNull());
   const Field& state_field = Field::ZoneHandle(
-      random_class.LookupInstanceField(Symbols::_state()));
+      random_class.LookupInstanceFieldAllowPrivate(Symbols::_state()));
   ASSERT(!state_field.IsNull());
   const Field& random_A_field = Field::ZoneHandle(
-      random_class.LookupStaticField(Symbols::_A()));
+      random_class.LookupStaticFieldAllowPrivate(Symbols::_A()));
   ASSERT(!random_A_field.IsNull());
   ASSERT(random_A_field.is_const());
   const Instance& a_value = Instance::Handle(random_A_field.StaticValue());