Copy-on-write arrays.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 7083224bdf3b16927d41c0b23ecd8dea4b2a4654..aedc3b9ced3416e8cfb83f828c88024a01cc6f53 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -832,6 +832,8 @@ class MacroAssembler: public Assembler {
   // Use --debug_code to enable.
   void Assert(Condition cc, const char* msg);
 
+  void AssertFastElements(Register elements);
+
   // Like Assert(), but always enabled.
   void Check(Condition cc, const char* msg);