[stubs] Port String.prototype.substr to TurboFan

src/builtins/builtins.h

Index: src/builtins/builtins.h
diff --git a/src/builtins/builtins.h b/src/builtins/builtins.h
index 6f11e6499cabe0a29f6a43618f6123419427584b..57135082fc155688dbcbcf1d801c62992f3cd959 100644
--- a/src/builtins/builtins.h
+++ b/src/builtins/builtins.h
@@ -545,6 +545,8 @@ namespace internal {
   CPP(StringPrototypeLocaleCompare)                                           \
   /* ES6 section 21.1.3.12 String.prototype.normalize ( [form] ) */           \
   CPP(StringPrototypeNormalize)                                               \
+  /* ES6 section B.2.3.1 String.prototype.substr ( start, length ) */         \
+  TFJ(StringPrototypeSubstr, 3)                                               \
   /* ES6 section 21.1.3.19 String.prototype.substring ( start, end ) */       \
   TFJ(StringPrototypeSubstring, 3)                                            \
   /* ES6 section 21.1.3.25 String.prototype.toString () */                    \