Use generated code to create sub strings

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
===================================================================
--- src/x64/codegen-x64.cc	(revision 3551)
+++ src/x64/codegen-x64.cc	(working copy)
@@ -3901,6 +3901,18 @@
 }
 
 
+void CodeGenerator::GenerateSubString(ZoneList<Expression*>* args) {
+  ASSERT_EQ(2, args->length());
+
+  Load(args->at(0));
+  Load(args->at(1));
+  Load(args->at(2));
+
+  Result answer = frame_->CallRuntime(Runtime::kSubString, 3);
+  frame_->Push(&answer);
+}
+
+
 void CodeGenerator::GenerateClassOf(ZoneList<Expression*>* args) {
   ASSERT(args->length() == 1);
   JumpTarget leave, null, function, non_function_constructor;