Add per-library cache of exported names

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index 011cf4e69db78ee33a1bd45f4cc6ea24016f53ad..40c1e3481e00a572db3a1acd493a3450e025b6cb 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -1182,6 +1182,7 @@ RawLibrary* Library::ReadFrom(SnapshotReader* reader,
     } else {
       library.InitResolvedNamesCache(kInitialNameCacheSize, reader);
     }
+    library.StorePointer(&library.raw_ptr()->exported_names_, Array::null());
     // Initialize cache of loaded scripts.
     library.StorePointer(&library.raw_ptr()->loaded_scripts_, Array::null());
   }