Fix throwing ObjectNotClosure instead of null exception if closure is null.

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
===================================================================
--- runtime/vm/code_generator.cc	(revision 1524)
+++ runtime/vm/code_generator.cc	(working copy)
@@ -772,7 +772,7 @@
   const Instance& bad_closure = Instance::CheckedHandle(arguments.At(0));
   if (bad_closure.IsNull()) {
     GrowableArray<const Object*> args;
-    Exceptions::ThrowByType(Exceptions::kNullPointer, args);
+    Exceptions::ThrowByType(Exceptions::kObjectNotClosure, args);
   }
   // const Array& arguments = Array::CheckedHandle(arguments.At(1));
   OS::PrintErr("object '%s' is not a closure\n", bad_closure.ToCString());