Trace signature types in the tree shaker.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index d7640482c0235ea3f5274477ca033a386ab12a82..84bbadc07756d5ca657ee18b9271578a35da0343 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -2205,6 +2205,7 @@ class Function : public Object {
   // function type with uninstantiated type arguments 'T' and 'R' as elements of
   // its type argument vector.
   RawType* SignatureType() const;
+  RawType* ExistingSignatureType() const;
 
   // Update the signature type (with a canonical version).
   void SetSignatureType(const Type& value) const;