Improve array to string conversion.

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 e102803c094d04d6c27af7f70df3aac591e9d9e1..b1bac1e6f50b9d7148b38111a36758894a4f34dc 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1581,6 +1581,17 @@ LInstruction* LChunkBuilder::DoDateField(HDateField* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoSeqStringSetChar(HSeqStringSetChar* instr) {
+  LOperand* string = UseRegister(instr->string());
+  LOperand* index = UseRegister(instr->index());
+  ASSERT(rcx.is_byte_register());
+  LOperand* value = UseFixed(instr->value(), rcx);
+  LSeqStringSetChar* result =
+      new(zone()) LSeqStringSetChar(instr->is_one_byte(), string, index, value);
+  return DefineSameAsFirst(result);
+}
+
+
 LInstruction* LChunkBuilder::DoBoundsCheck(HBoundsCheck* instr) {
   LOperand* value = UseRegisterOrConstantAtStart(instr->index());
   LOperand* length = Use(instr->length());