Fix building of parameter descriptors from Kernel

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index 89d4f7b2fdab88d282c649838118d1d56190cab7..740679677e73b7c07fcff69e05cce55435fdc6df 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -145,11 +145,8 @@ static RawInstance* CreateParameterMirrorList(const Function& func,
     // * Whether a parameters has been declared as final.
     // * Any metadata associated with the parameter.
     Object& result = Object::Handle();
-
-    kernel::TreeNode* kernel_node =
-        reinterpret_cast<kernel::TreeNode*>(func.kernel_function());
-    if (kernel_node != NULL) {
-      result = kernel::BuildParameterDescriptor(kernel_node);
+    if (func.kernel_function() != NULL) {
+      result = kernel::BuildParameterDescriptor(func);
     } else {
       result = Parser::ParseFunctionParameters(func);
     }