Don't flatten every time we call CharCodeAt

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 088c2478e6d3229033fd3e34984f9e77c8d055de..4ab04cca1cc87552dc75f090e33e8891a9e38ecd 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -994,6 +994,14 @@ class Isolate {
 
   void ResetEagerOptimizingData();
 
+  bool ShouldFlattenString() {
+    if (--flatten_counter_ != 0) {
+      return false;
+    }
+    flatten_counter_ = kStringFlattenInterval;
+    return true;
+  }
+
  private:
   Isolate();
 
@@ -1157,6 +1165,9 @@ class Isolate {
   ZoneObjectList frame_element_constant_list_;
   ZoneObjectList result_constant_list_;
 
+  static const int kStringFlattenInterval = 257;
+  int flatten_counter_;
+
 #if defined(V8_TARGET_ARCH_ARM) && !defined(__arm__) || \
     defined(V8_TARGET_ARCH_MIPS) && !defined(__mips__)
   bool simulator_initialized_;