Reland "Handlify concat string and substring."

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 92086d4b304d1e477937e4712ce7bc69d40ca24d..94e89f58a7aa10e7afbd47bd9128c02d30afe6c6 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -158,23 +158,28 @@ class Factory {
       PretenureFlag pretenure = NOT_TENURED);
 
   // Create a new cons string object which consists of a pair of strings.
-  Handle<String> NewConsString(Handle<String> first,
-                               Handle<String> second);
+  Handle<String> NewConsString(Handle<String> left,
+                               Handle<String> right);
+
+  Handle<ConsString> NewRawConsString(String::Encoding encoding);
 
   // 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,
-                              int end);
-
   // Create a new string object which holds a proper substring of a string.
   Handle<String> NewProperSubString(Handle<String> str,
                                     int begin,
                                     int end);
 
+  // Create a new string object which holds a substring of a string.
+  Handle<String> NewSubString(Handle<String> str, int begin, int end) {
+    if (begin == 0 && end == str->length()) return str;
+    return NewProperSubString(str, begin, end);
+  }
+
+  Handle<SlicedString> NewRawSlicedString(String::Encoding encoding);
+
   // Creates a new external String object.  There are two String encodings
   // in the system: ASCII and two byte.  Unlike other String types, it does
   // not make sense to have a UTF-8 factory function for external strings,