Reduce snapshot size by not writing null values for function code fields.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 37241)
+++ runtime/vm/raw_object.h	(working copy)
@@ -620,6 +620,9 @@
   RawObject** to_no_code() {
     return reinterpret_cast<RawObject**>(&ptr()->data_);
   }
+  RawObject** to_snapshot() {
+    return reinterpret_cast<RawObject**>(&ptr()->data_);
+  }
 
   intptr_t token_pos_;
   intptr_t end_token_pos_;