Use symbols when looking up fields in a class

runtime/vm/guard_field_test.cc

 // Copyright (c) 2013, 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/dart_api_impl.h"
 #include "vm/dart_api_state.h"
 #include "vm/object.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 RawField* LookupField(Dart_Handle library, const char* class_name,
                       const char* field_name) {
   RawLibrary* raw_library = Library::RawCast(Api::UnwrapHandle(library));
   Library& lib = Library::ZoneHandle(raw_library);
   const String& classname = String::Handle(Symbols::New(class_name));
   Class& cls = Class::Handle(lib.LookupClass(classname));
   EXPECT(!cls.IsNull());  // No ambiguity error expected.
 
   String& fieldname = String::Handle(String::New(field_name));
-  Field& field = Field::ZoneHandle(cls.LookupInstanceField(fieldname));
+  Field& field =
+      Field::ZoneHandle(cls.LookupInstanceFieldAllowPrivate(fieldname));
   EXPECT(!field.IsNull());
   return field.raw();
 }
 
 
 TEST_CASE(GuardFieldSimpleTest) {
   const char* script_chars =
       "class A {\n"
       "  var f1 = 3.0;\n"
       "  var f2 = 3;\n"
@@ skipping 240 matching lines @@
   Dart_Handle result = Dart_Invoke(lib, NewString("main"), 0, NULL);
   EXPECT_VALID(result);
   Field& f3 = Field::ZoneHandle(LookupField(lib, "A", "f3"));
   const intptr_t no_length = Field::kNoFixedLength;
   EXPECT_EQ(no_length, f3.guarded_list_length());
   EXPECT_EQ(kTypedDataFloat32ArrayCid, f3.guarded_cid());
   EXPECT_EQ(false, f3.is_nullable());
 }
 
 }  // namespace dart