Add initial hydrogenized NewStringAddStub.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index b5046bd00190a46b4ae0aecc5c804b10989afc54..d1c85d9b15c001be604549ce3c31ea3199040e3a 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -1263,6 +1263,23 @@ class HGraphBuilder {
 
   HValue* BuildNumberToString(HValue* object, Handle<Type> type);
 
+  // Copies characters from one sequential string to another.
+  void BuildCopySeqStringChars(HValue* src,
+                               HValue* src_offset,
+                               String::Encoding src_encoding,
+                               HValue* dst,
+                               HValue* dst_offset,
+                               String::Encoding dst_encoding,
+                               HValue* length);
+  // Both operands are non-empty strings.
+  HValue* BuildUncheckedStringAdd(HValue* left,
+                                  HValue* right,
+                                  PretenureFlag pretenure_flag);
+  // Both operands are strings.
+  HValue* BuildStringAdd(HValue* left,
+                         HValue* right,
+                         PretenureFlag pretenure_flag);
+
   HInstruction* BuildUncheckedMonomorphicElementAccess(
       HValue* checked_object,
       HValue* key,