Deletion barrier preparation: validate overwritten references.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 42583)
+++ runtime/vm/pages.h	(working copy)
@@ -323,6 +323,11 @@
   uword TryAllocateDataBumpLocked(intptr_t size, GrowthPolicy growth_policy);
   // Prefer small freelist blocks, then chip away at the bump block.
   uword TryAllocatePromoLocked(intptr_t size, GrowthPolicy growth_policy);
+  // Allocates memory where every word is guaranteed to be a Smi. Calling this
+  // method after the first garbage collection is inefficient in release mode
+  // and illegal in debug mode.
+  uword TryAllocateSmiInitializedLocked(intptr_t size,
+                                        GrowthPolicy growth_policy);
 
   // Bump block allocation from generated code.
   uword* TopAddress() { return &bump_top_; }