Add initial hydrogenized NewStringAddStub.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index 8f4878d93e9c065bad1b36579099cd64d08fa8c9..b3cb8ffc75dc3796ae288967b29c9c46f21a2e3e 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -1276,6 +1276,26 @@ class HGraphBuilder {
 
   HValue* BuildNumberToString(HValue* object, Handle<Type> type);
 
+  // Computes the size for a sequential string of the given length and encoding.
+  HValue* BuildSeqStringSizeFor(HValue* length,
+                                String::Encoding encoding);
+  // 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,
@@ -1298,7 +1318,13 @@ class HGraphBuilder {
   HLoadNamedField* BuildLoadNamedField(HValue* object, HObjectAccess access);
   HInstruction* AddLoadNamedField(HValue* object, HObjectAccess access);
   HInstruction* BuildLoadStringLength(HValue* object, HValue* checked_value);
-  HStoreNamedField* AddStoreMapConstant(HValue* object, Handle<Map>);
+  HStoreNamedField* AddStoreMapConstant(HValue* object, Handle<Map> map);
+  HStoreNamedField* AddStoreMapConstantNoWriteBarrier(HValue* object,
+                                                      Handle<Map> map) {
+    HStoreNamedField* store_map = AddStoreMapConstant(object, map);
+    store_map->SkipWriteBarrier();
+    return store_map;
+  }
   HLoadNamedField* AddLoadElements(HValue* object);
 
   bool MatchRotateRight(HValue* left,