Remove signature classes from the VM.

runtime/vm/debugger_api_impl.cc

Index: runtime/vm/debugger_api_impl.cc
diff --git a/runtime/vm/debugger_api_impl.cc b/runtime/vm/debugger_api_impl.cc
index 7e1ec45e9616b465f7c549a9bb3d3e5b9f36010b..31cd394f4a197880935c2fd4dd594f9d9448518f 100644
--- a/runtime/vm/debugger_api_impl.cc
+++ b/runtime/vm/debugger_api_impl.cc
@@ -671,7 +671,7 @@ DART_EXPORT Dart_Handle Dart_GetClosureInfo(
   if (!instance.IsClosure()) {
     return Api::NewError("%s: parameter 0 is not a closure", CURRENT_FUNC);
   }
-  const Function& func = Function::Handle(Closure::function(instance));
+  const Function& func = Function::Handle(Closure::Cast(instance).function());
   ASSERT(!func.IsNull());
   if (name != NULL) {
     *name = Api::NewHandle(T, func.QualifiedUserVisibleName());