Set and keep parent function of signature functions.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index 13ac6751d3612cac1c4d2e1a3ea87e1228f3a7be..f6d693b4c28aa1e478c73234cce9cc15e8e29a4f 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -591,7 +591,11 @@ void ClassFinalizer::ResolveType(const Class& cls, const AbstractType& type) {
           // The parent function, owner, and token position of a shared
           // canonical function type are meaningless, since the canonical
           // representent is picked arbitrarily.
-          signature.set_parent_function(Function::Handle());
+          // The parent function is however still required to finalize function
+          // type parameters when the function has a generic parent.
+          if (!signature.HasGenericParent()) {
+            signature.set_parent_function(Function::Handle());
+          }
           // TODO(regis): As long as we support metadata in typedef signatures,
           // we cannot reset these fields used to reparse a typedef.
           // Note that the scope class of a typedef function type is always