VM: Fix bug in CHA for signature classes.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
===================================================================
--- runtime/vm/class_finalizer.cc	(revision 38967)
+++ runtime/vm/class_finalizer.cc	(working copy)
@@ -2206,6 +2206,13 @@
     // Resolve and finalize the signature type of this signature class.
     const Type& sig_type = Type::Handle(cls.SignatureType());
     FinalizeType(cls, sig_type, kCanonicalizeWellFormed);
+
+    // Add this class to the subclasses of the superclass (_FunctionImpl).
+    if (!super_type.IsNull()) {
+      ASSERT(!super_type.IsObjectType());
+      ASSERT(!super_class.IsNull());
+      super_class.AddDirectSubclass(cls);
+    }
     return;
   }
   // Finalize interface types (but not necessarily interface classes).