Use the app snapshot when testing with -cdartk.

runtime/vm/clustered_snapshot.cc

Index: runtime/vm/clustered_snapshot.cc
diff --git a/runtime/vm/clustered_snapshot.cc b/runtime/vm/clustered_snapshot.cc
index 9929c3d49f4a97fecd9400eb4fba79108b56bdd1..6ab2c9ab3f376b4e6665f9db1702e7fc9cabff67 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -4647,7 +4647,7 @@ void Serializer::WriteVersionAndFeatures() {
   const intptr_t version_len = strlen(expected_version);
   WriteBytes(reinterpret_cast<const uint8_t*>(expected_version), version_len);
 
-  const char* expected_features = Dart::FeaturesString(kind_);
+  const char* expected_features = Snapshot::FeaturesString(kind_);
   ASSERT(expected_features != NULL);
   const intptr_t features_len = strlen(expected_features);
   WriteBytes(reinterpret_cast<const uint8_t*>(expected_features),
@@ -5021,7 +5021,7 @@ RawApiError* Deserializer::VerifyVersionAndFeatures() {
   }
   Advance(version_len);
 
-  const char* expected_features = Dart::FeaturesString(kind_);
+  const char* expected_features = Snapshot::FeaturesString(kind_);
   ASSERT(expected_features != NULL);
   const intptr_t expected_len = strlen(expected_features);