Use VM type cast and save handles.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
===================================================================
--- runtime/vm/debugger.cc	(revision 9315)
+++ runtime/vm/debugger.cc	(working copy)
@@ -655,12 +655,11 @@
                         const Library& library,
                         const String& fname) {
   ASSERT(!library.IsNull());
-  Function& function = Function::Handle();
   const Object& object = Object::Handle(library.LookupObject(fname));
   if (!object.IsNull() && object.IsFunction()) {
-    function ^= object.raw();
+    return Function::Cast(object).raw();
   }
-  return function.raw();
+  return Function::null();
 }