Support string add in crankshaft:

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 49124497fba7dc5566a77950321e6858ff40f0df..d4b651def8c0a5eeb3eeb6e860aaab8a85aa4469 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -3223,6 +3223,14 @@ void LCodeGen::DoStoreKeyedGeneric(LStoreKeyedGeneric* instr) {
 }
 
 
+void LCodeGen::DoStringAdd(LStringAdd* instr) {
+  __ push(ToRegister(instr->left()));
+  __ push(ToRegister(instr->right()));
+  StringAddStub stub(NO_STRING_CHECK_IN_STUB);
+  CallCode(stub.GetCode(), RelocInfo::CODE_TARGET, instr);
+}
+
+
 void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   class DeferredStringCharCodeAt: public LDeferredCode {
    public: