Introduce a SignatureData class so that signature functions can store both their

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
diff --git a/runtime/vm/raw_object.cc b/runtime/vm/raw_object.cc
index 79a6193460c2bc7e1543f86b34c38e3e212c0971..f4a1bc17618dd1f7de376c95dd4a86f1ee78eac0 100644
--- a/runtime/vm/raw_object.cc
+++ b/runtime/vm/raw_object.cc
@@ -393,6 +393,14 @@ intptr_t RawClosureData::VisitClosureDataPointers(
 }
 
 
+intptr_t RawSignatureData::VisitSignatureDataPointers(
+    RawSignatureData* raw_obj,
+    ObjectPointerVisitor* visitor) {
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return SignatureData::InstanceSize();
+}
+
+
 intptr_t RawRedirectionData::VisitRedirectionDataPointers(
     RawRedirectionData* raw_obj,
     ObjectPointerVisitor* visitor) {