ia32 StringAddStub: fast two character case.

src/ia32/macro-assembler-ia32.h

Index: src/ia32/macro-assembler-ia32.h
diff --git a/src/ia32/macro-assembler-ia32.h b/src/ia32/macro-assembler-ia32.h
index 5e850c01cbd55324dc08affa55c39cdbd9530728..c3afc033372233baa10d7128a5948b1ecdb6f615 100644
--- a/src/ia32/macro-assembler-ia32.h
+++ b/src/ia32/macro-assembler-ia32.h
@@ -356,6 +356,10 @@ class MacroAssembler: public Assembler {
                            Register scratch2,
                            Register scratch3,
                            Label* gc_required);
+  void AllocateAsciiString(Register result,
+                           int length,
+                           Register scratch,
+                           Label* gc_required);
 
   // Allocate a raw cons string object. Only the map field of the result is
   // initialized.