ART: Add dex::StringIndex

Add abstraction for uint32_t string index.

Test: m test-art-host
Change-Id: I917c2881702fe3df112c713f06980f2278ced7ed
diff --git a/dexlist/dexlist.cc b/dexlist/dexlist.cc
index 68473c4..efe1aad 100644
--- a/dexlist/dexlist.cc
+++ b/dexlist/dexlist.cc
@@ -140,7 +140,7 @@
   const DexFile::ClassDef& pClassDef = pDexFile->GetClassDef(idx);
 
   const char* fileName;
-  if (pClassDef.source_file_idx_ == DexFile::kDexNoIndex) {
+  if (!pClassDef.source_file_idx_.IsValid()) {
     fileName = nullptr;
   } else {
     fileName = pDexFile->StringDataByIdx(pClassDef.source_file_idx_);