Use symbols when looking up fields in a class

runtime/vm/exceptions.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/exceptions.h"
 
 #include "platform/address_sanitizer.h"
 
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
@@ skipping 252 matching lines @@
     const Library& core_lib = Library::Handle(zone, Library::CoreLibrary());
     error_class = core_lib.LookupClass(Symbols::Error());
     ASSERT(!error_class.IsNull());
     isolate->object_store()->set_error_class(error_class);
   }
   // If instance class extends 'class Error' return '_stackTrace' field.
   Class& test_class = Class::Handle(zone, instance.clazz());
   AbstractType& type = AbstractType::Handle(zone, AbstractType::null());
   while (true) {
     if (test_class.raw() == error_class.raw()) {
-      return error_class.LookupInstanceField(Symbols::_stackTrace());
+      return error_class.LookupInstanceFieldAllowPrivate(
+          Symbols::_stackTrace());
     }
     type = test_class.super_type();
     if (type.IsNull()) return Field::null();
     test_class = type.type_class();
   }
   UNREACHABLE();
   return Field::null();
 }
 
 
@@ skipping 427 matching lines @@
   }
 
   return DartLibraryCalls::InstanceCreate(library,
                                           *class_name,
                                           *constructor_name,
                                           arguments);
 }
 
 
 }  // namespace dart