Rollback of r15267, r15263, r15193 in trunk branch in preparation for 3.19 branch

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 9ea66bee7b45fbb8ff171ece415010bde56593da..cde84325dcf755c059618672157427a97dc97c7e 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -152,10 +152,6 @@ class Factory {
   Handle<String> NewConsString(Handle<String> first,
                                Handle<String> second);
 
-  // Create a new sequential string containing the concatenation of the inputs.
-  Handle<String> NewFlatConcatString(Handle<String> first,
-                                     Handle<String> second);
-
   // Create a new string object which holds a substring of a string.
   Handle<String> NewSubString(Handle<String> str,
                               int begin,