[builtins] String.prototype.slice as a CSA builtin.

src/code-stub-assembler.h

Index: src/code-stub-assembler.h
diff --git a/src/code-stub-assembler.h b/src/code-stub-assembler.h
index a7f3ef8afaed105aca09f8b9aed215805aa10fb9..ec71d0a2aea29e8da87bd0146983d52da1ea07ea 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -802,9 +802,15 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
                          ParameterMode parameter_mode = SMI_PARAMETERS);
   // Return the single character string with only {code}.
   Node* StringFromCharCode(Node* code);
+
+  enum class SubStringFlags { NONE, FROM_TO_ARE_BOUNDED };
+
   // Return a new string object which holds a substring containing the range
   // [from,to[ of string.  |from| and |to| are expected to be tagged.
-  Node* SubString(Node* context, Node* string, Node* from, Node* to);
+  // If flags has the value FROM_TO_ARE_BOUNDED then from and to are in
+  // the range [0, string-length)
+  Node* SubString(Node* context, Node* string, Node* from, Node* to,
+                  SubStringFlags flags = SubStringFlags::NONE);
 
   // Return a new string object produced by concatenating |first| with |second|.
   Node* StringAdd(Node* context, Node* first, Node* second,