Fix 15721: When checking for bad overrides we must eagerly finalize superclasses.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
===================================================================
--- runtime/vm/class_finalizer.cc	(revision 31621)
+++ runtime/vm/class_finalizer.cc	(working copy)
@@ -1273,6 +1273,9 @@
       // A constructor cannot override anything.
       for (intptr_t i = 0; i < interfaces.Length(); i++) {
         super_class ^= interfaces.At(i);
+        // Finalize superclass since overrides check relies on all members
+        // of the superclass to be finalized.
+        FinalizeClass(super_class);
         overridden_function = super_class.LookupDynamicFunction(name);
         if (!overridden_function.IsNull() &&
             !function.HasCompatibleParametersWith(overridden_function,