Reland "Handlify concat string and substring."

src/handles.h

 // Copyright 2011 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 227 matching lines @@
 Handle<Object> ForceDeleteProperty(Handle<JSObject> object, Handle<Object> key);
 
 Handle<Object> HasProperty(Handle<JSReceiver> obj, Handle<Object> key);
 
 Handle<Object> GetProperty(Handle<JSReceiver> obj, const char* name);
 
 Handle<Object> GetProperty(Isolate* isolate,
                            Handle<Object> obj,
                            Handle<Object> key);
 
-Handle<Object> LookupSingleCharacterStringFromCode(Isolate* isolate,
+Handle<String> LookupSingleCharacterStringFromCode(Isolate* isolate,
                                                    uint32_t index);
 
 Handle<FixedArray> AddKeysFromJSArray(Handle<FixedArray>,
                                       Handle<JSArray> array);
 
 // Get the JS object corresponding to the given script; create it
 // if none exists.
 Handle<JSValue> GetScriptWrapper(Handle<Script> script);
 
 // Script line number computations. Note that the line number is zero-based.
@@ skipping 54 matching lines @@
  private:
   Isolate* isolate_;
   Object** limit_;
   int level_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_HANDLES_H_