Stack Trace Element with its unit test added too.

The unit test passes.

Change-Id: I6f6af771cae1387c60c3b511148a347fe3237345
diff --git a/src/dex_file.h b/src/dex_file.h
index 2dcb3b2..db7a54b 100644
--- a/src/dex_file.h
+++ b/src/dex_file.h
@@ -535,7 +535,7 @@
     return dexStringById(idx, &unicode_length);
   }
 
-  String* dexArtStringById(uint32_t idx) const;
+  String* dexArtStringById(int32_t idx) const;
 
   // Get the descriptor string associated with a given type index.
   const char* dexStringByTypeIdx(uint32_t idx, int32_t* unicode_length) const {
@@ -548,7 +548,7 @@
     return dexStringById(type_id.descriptor_idx_);
   }
 
-  String* dexArtStringByTypeIdx(uint32_t idx) const {
+  String* dexArtStringByTypeIdx(int32_t idx) const {
     const TypeId& type_id = GetTypeId(idx);
     return dexArtStringById(type_id.descriptor_idx_);
   }