Revert 22595: "Keep new arrays allocated with 'new Array(N)' in fast mode"

src/elements.cc

Index: src/elements.cc
diff --git a/src/elements.cc b/src/elements.cc
index fc0f3b8f43ebfdb243d2dc58e2f093680a6ebbec..3ef6bfd0e1130920d885831d50a00b838173a13d 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -897,7 +897,8 @@ class FastElementsAccessor
 
   typedef typename KindTraits::BackingStore BackingStore;
 
-  // Adjusts the length of the fast backing store..
+  // Adjusts the length of the fast backing store or returns the new length or
+  // undefined in case conversion to a slow backing store should be performed.
   static Handle<Object> SetLengthWithoutNormalize(
       Handle<FixedArrayBase> backing_store,
       Handle<JSArray> array,
@@ -949,10 +950,15 @@ class FastElementsAccessor
     // Check whether the backing store should be expanded.
     uint32_t min = JSObject::NewElementsCapacity(old_capacity);
     uint32_t new_capacity = length > min ? length : min;
-    FastElementsAccessorSubclass::SetFastElementsCapacityAndLength(
-        array, new_capacity, length);
-    JSObject::ValidateElements(array);
-    return length_object;
+    if (!array->ShouldConvertToSlowElements(new_capacity)) {
+      FastElementsAccessorSubclass::SetFastElementsCapacityAndLength(
+          array, new_capacity, length);
+      JSObject::ValidateElements(array);
+      return length_object;
+    }
+
+    // Request conversion to slow elements.
+    return isolate->factory()->undefined_value();
   }
 
   static Handle<Object> DeleteCommon(Handle<JSObject> obj,