Add initial hydrogenized NewStringAddStub.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index bf8b7b977d50bd181465298e13a6067ce8bc12e5..5c346cb248e54da1428fc34a90a09c36862a116c 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -4582,10 +4582,18 @@ void LCodeGen::DoTrapAllocationMemento(LTrapAllocationMemento* instr) {
 
 void LCodeGen::DoStringAdd(LStringAdd* instr) {
   ASSERT(ToRegister(instr->context()).is(cp));
-  __ push(ToRegister(instr->left()));
-  __ push(ToRegister(instr->right()));
-  StringAddStub stub(instr->hydrogen()->flags());
-  CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
+  if (FLAG_new_string_add) {
+    ASSERT(ToRegister(instr->left()).is(r1));
+    ASSERT(ToRegister(instr->right()).is(r0));
+    NewStringAddStub stub(instr->hydrogen()->flags(),
+                          isolate()->heap()->GetPretenureMode());
+    CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
+  } else {
+    __ push(ToRegister(instr->left()));
+    __ push(ToRegister(instr->right()));
+    StringAddStub stub(instr->hydrogen()->flags());
+    CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
+  }
 }