Make sure all new Code objects have a non-null PcDescriptors

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index a41379387f8a9c0e7bf9e17e9fb88b2125b5d349..2a107dbc680d08490de9a1d8bedfa98e0eaa61dd 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -368,6 +368,11 @@ class Object {
     return *empty_array_;
   }
 
+  static const PcDescriptors& empty_descriptors() {
+    ASSERT(empty_descriptors_ != NULL);
+    return *empty_descriptors_;
+  }
+
   // The sentinel is a value that cannot be produced by Dart code.
   // It can be used to mark special values, for example to distinguish
   // "uninitialized" fields.
@@ -614,6 +619,7 @@ class Object {
   static Instance* null_instance_;
   static AbstractTypeArguments* null_abstract_type_arguments_;
   static Array* empty_array_;
+  static PcDescriptors* empty_descriptors_;
   static Instance* sentinel_;
   static Instance* transition_sentinel_;
   static Instance* unknown_constant_;
@@ -2992,6 +2998,7 @@ class PcDescriptors : public Object {
 
   FINAL_HEAP_OBJECT_IMPLEMENTATION(PcDescriptors, Object);
   friend class Class;
+  friend class Object;
 };