Improve fast to slow elements conversion:

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 9b55ea747545799a7f35aa4948bdcfad46dccd0b..020019da978d9ddd65f61028834bb6611fb2a147 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1946,8 +1946,21 @@ class JSObject: public JSReceiver {
   // Also maximal value of JSArray's length property.
   static const uint32_t kMaxElementCount = 0xffffffffu;
 
+  // Constants for heuristics controlling conversion of fast elements
+  // to slow elements.
+
+  // Maximal gap that can be introduced by adding an element beyond
+  // the current elements length.
   static const uint32_t kMaxGap = 1024;
-  static const int kMaxFastElementsLength = 5000;
+
+  // Maximal length of fast elements array that won't be checked for
+  // being dense enough on expansion.
+  static const int kMaxUncheckedFastElementsLength = 5000;
+
+  // Same as above but for old arrays. This limit is more strict. We
+  // don't want to be wasteful with long lived objects.
+  static const int kMaxUncheckedOldFastElementsLength = 500;
+
   static const int kInitialMaxFastElementArray = 100000;
   static const int kMaxFastProperties = 12;
   static const int kMaxInstanceSize = 255 * kPointerSize;
@@ -2013,6 +2026,9 @@ class JSObject: public JSReceiver {
   // Returns true if most of the elements backing storage is used.
   bool HasDenseElements();
 
+  // Gets the current elements capacity and the number of used elements.
+  void GetElementsCapacityAndUsage(int* capacity, int* used);
+
   bool CanSetCallback(String* name);
   MUST_USE_RESULT MaybeObject* SetElementCallback(
       uint32_t index,