Introduce class TypeRef in the VM to fully support recursive types.

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
===================================================================
--- runtime/vm/raw_object.cc	(revision 31085)
+++ runtime/vm/raw_object.cc	(working copy)
@@ -301,6 +301,13 @@
 }
 
 
+intptr_t RawTypeRef::VisitTypeRefPointers(
+    RawTypeRef* raw_obj, ObjectPointerVisitor* visitor) {
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return TypeRef::InstanceSize();
+}
+
+
 intptr_t RawTypeParameter::VisitTypeParameterPointers(
     RawTypeParameter* raw_obj, ObjectPointerVisitor* visitor) {
   visitor->VisitPointers(raw_obj->from(), raw_obj->to());