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

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index b3c087d9ea2a18b905200b9146ae31da8ffbd543..76f48a6f0f42e269cce1f95d7284a5c07ea2d12c 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -77,6 +77,7 @@ class RawPcDescriptors;
 class RawReceivePort;
 class RawRedirectionData;
 class RawScript;
+class RawSignatureData;
 class RawSendPort;
 class RawSmi;
 class RawStackMap;
@@ -542,6 +543,7 @@ class SnapshotReader : public BaseReader {
   friend class PatchClass;
   friend class RedirectionData;
   friend class Script;
+  friend class SignatureData;
   friend class SubtypeTestCache;
   friend class TokenStream;
   friend class Type;