This adds new methods to String object:

src/bootstrapper.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2061 matching lines @@
     if (FLAG_harmony_generators &&
         strcmp(ExperimentalNatives::GetScriptName(i).start(),
                "native generator.js") == 0) {
       if (!CompileExperimentalBuiltin(isolate(), i)) return false;
     }
     if (FLAG_harmony_iteration &&
         strcmp(ExperimentalNatives::GetScriptName(i).start(),
                "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();
 
   return true;
 }
 
 
 static Handle<JSObject> ResolveBuiltinIdHolder(
     Handle<Context> native_context,
@@ skipping 582 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   ASSERT(!IsActive());
 }
 
 } }  // namespace v8::internal