Flatten before String::WriteUtf8

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index f64b3867367ab1f65729b04c4b6666c1a385fc1c..e81e2e57fa9793b5d900ea210ece917e9b4ab53b 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -863,6 +863,13 @@ class V8EXPORT String : public Primitive {
   int WriteUtf8(char* buffer, int length = -1) const; // UTF-8
 
   /**
+   * Flatten internal memory. Operations on the string tend to run faster
+   * after flattening especially if the string is a concatenation of many
+   * others.
+   */
+  void Flatten();
+
+  /**
    * A zero length string.
    */
   static v8::Local<v8::String> Empty();