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 0ca54d8f33c2282146cd312db1cc2547aa36f4cf..9d7eaa175997dcd75e3e1a32a95a0e2ed96e0a14 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());
   }