Teach treeshaker about new cached functions and classes.

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 1ab549434c4d99f7da6b09235d2c03f52bf68ad1..6faedb87a23dae15fbf864d935d0e3cc658d68dc 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -113,7 +113,10 @@ class ClassSerializationCluster : public SerializationCluster {
       s->WriteRef(*p);
     }
     intptr_t class_id = cls->ptr()->id_;
-    ASSERT(class_id != kIllegalCid);
+    if (class_id == kIllegalCid) {
+      FATAL1("Attempting to serialize class with illegal cid: %s\n",
+             Class::Handle(cls).ToCString());
+    }
     s->WriteCid(class_id);
     s->Write<int32_t>(cls->ptr()->instance_size_in_words_);
     s->Write<int32_t>(cls->ptr()->next_field_offset_in_words_);