Tentative implementation of string slices (hidden under the flag --string-slices).

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 5b502a80c91b1ca40226c17d08e2d0786897fe1f..357fa7e274b5a2f8082a8c95425ce84dfd8dcfdb 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1991,7 +1991,8 @@ LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
   LOperand* string = UseRegister(instr->string());
   LOperand* index = UseRegisterOrConstant(instr->index());
-  LStringCharCodeAt* result = new LStringCharCodeAt(string, index);
+  LOperand* temp = TempRegister();
+  LStringCharCodeAt* result = new LStringCharCodeAt(string, index, temp);
   return AssignEnvironment(AssignPointerMap(DefineAsRegister(result)));
 }