Fix debugging output for unresolved fields.

Change-Id: I539396004457e63981bc4f5f5b08e4a2365952ce
diff --git a/src/compiler/codegen/arm/Thumb2/Gen.cc b/src/compiler/codegen/arm/Thumb2/Gen.cc
index 0db63f9..1ce7394 100644
--- a/src/compiler/codegen/arm/Thumb2/Gen.cc
+++ b/src/compiler/codegen/arm/Thumb2/Gen.cc
@@ -32,7 +32,7 @@
     const art::DexFile& dex_file = class_linker->FindDexFile(
          method->GetDeclaringClass()->GetDexCache());
     const art::DexFile::FieldId& field_id = dex_file.GetFieldId(fieldIdx);
-    std::string class_name = dex_file.dexStringById(field_id.class_idx_);
+    std::string class_name = dex_file.dexStringByTypeIdx(field_id.class_idx_);
     std::string field_name = dex_file.dexStringById(field_id.name_idx_);
     return class_name + "." + field_name;
 }