Fix test failure issue 7471: Signature classes have different type checking rules, i.e., they have …

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
===================================================================
--- runtime/vm/flow_graph_compiler.cc	(revision 16330)
+++ runtime/vm/flow_graph_compiler.cc	(working copy)
@@ -1020,6 +1020,8 @@
   if (!FLAG_use_cha || !is_optimizing()) return false;
   if (!type.IsInstantiated()) return false;
   const Class& type_class = Class::Handle(type.type_class());
+  // Signature classes have different type checking rules.
+  if (type_class.IsSignatureClass()) return false;
   // Could be an interface check?
   if (type_class.is_implemented()) return false;
   const intptr_t type_cid = type_class.id();