Re-enable assert to check canonicalization of signature type after snapshot

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index ab5d16f919665d6c44817b18ae2ff7df971d84a6..09926111a8c97c3f676a9b8b830107baaf4edf00 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -121,11 +121,6 @@ class FunctionVisitor : public ObjectVisitor {
   void VisitObject(RawObject* obj) {
     if (obj->IsFunction()) {
       funcHandle_ ^= obj;
-      if (funcHandle_.IsSignatureFunction()) {
-        // TODO(27606): Remove signature function case.
-        return;
-      }
-
       classHandle_ ^= funcHandle_.Owner();
       // Verify that the result type of a function is canonical or a
       // TypeParameter.