This adds new methods to String object:

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index dda5fe42a5a503cc99b7d753f8ed6895fae8c4e3..1748128701a55fded6e99c4fa1ea4e8d8fd76c07 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -2079,6 +2079,11 @@ bool Genesis::InstallExperimentalNatives() {
                "native array-iterator.js") == 0) {
       if (!CompileExperimentalBuiltin(isolate(), i)) return false;
     }
+    if (FLAG_harmony_string &&
+        strcmp(ExperimentalNatives::GetScriptName(i).start(),
+               "native harmony-string.js") == 0) {
+      if (!CompileExperimentalBuiltin(isolate(), i)) return false;
+    }
   }
 
   InstallExperimentalNativeFunctions();