Support redirecting factory constructors in the VM (issue 3969).

runtime/vm/class_finalizer.h

Index: runtime/vm/class_finalizer.h
===================================================================
--- runtime/vm/class_finalizer.h	(revision 12790)
+++ runtime/vm/class_finalizer.h	(working copy)
@@ -80,6 +80,10 @@
                                 const UnresolvedClass& unresolved_class);
   static void ResolveSuperType(const Class& cls);
   static void ResolveFactoryClass(const Class& cls);
+  static void ResolveRedirectingFactoryTarget(
+      const Class& cls,
+      const Function& factory,
+      const GrowableObjectArray& visited_factories);
   static void ResolveInterfaces(const Class& cls,
                                 GrowableArray<intptr_t>* visited);
   static void FinalizeTypeParameters(const Class& cls);