[stubs] Port String.prototype.substr to TurboFan

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 1c42a117bcb7dda52d8f6b638c69030832fec6c6..2f4d945c2ca9a2e5d279802cf420d7647fb6572f 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1408,6 +1408,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
                           Builtins::kStringPrototypeLocaleCompare, 1, true);
     SimpleInstallFunction(prototype, "normalize",
                           Builtins::kStringPrototypeNormalize, 0, false);
+    SimpleInstallFunction(prototype, "substr", Builtins::kStringPrototypeSubstr,
+                          2, true);
     SimpleInstallFunction(prototype, "substring",
                           Builtins::kStringPrototypeSubstring, 2, true);
     SimpleInstallFunction(prototype, "toString",