JSArray can go slow case during String.split. Guard against

src/runtime.cc

 // 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 6073 matching lines @@
   for (int i = 0; i < part_count; i++) {
     HandleScope local_loop_handle;
     int part_end = indices.at(i);
     Handle<String> substring =
         isolate->factory()->NewProperSubString(subject, part_start, part_end);
     elements->set(i, *substring);
     part_start = part_end + pattern_length;
   }
 
   if (limit == 0xffffffffu) {
-    StringSplitCache::Enter(isolate->heap(),
-                            isolate->heap()->string_split_cache(),
-                            *subject,
-                            *pattern,
-                            *elements);
+    if (result->HasFastElements()) {
+      StringSplitCache::Enter(isolate->heap(),
+                              isolate->heap()->string_split_cache(),
+                              *subject,
+                              *pattern,
+                              *elements);
+    }
   }
 
   return *result;
 }
 
 
 // Copies ascii characters to the given fixed array looking up
 // one-char strings in the cache. Gives up on the first char that is
 // not in the cache and fills the remainder with smi zeros. Returns
 // the length of the successfully copied prefix.
@@ skipping 6913 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal