Support old-space allocation in generated code (bump block only for now).

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 40518)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -871,14 +871,14 @@
   }
 
   void UpdateAllocationStats(intptr_t cid,
-                             Heap::Space space = Heap::kNew);
+                             Heap::Space space);
 
   void UpdateAllocationStatsWithSize(intptr_t cid,
                                      Register size_reg,
-                                     Heap::Space space = Heap::kNew);
+                                     Heap::Space space);
   void UpdateAllocationStatsWithSize(intptr_t cid,
                                      intptr_t instance_size,
-                                     Heap::Space space = Heap::kNew);
+                                     Heap::Space space);
 
   // Inlined allocation of an instance of class 'cls', code has no runtime
   // calls. Jump to 'failure' if the instance cannot be allocated here.