Protect the emptiness of Array prototype elements with a PropertyCell.

test/mjsunit/concurrent-initial-prototype-change.js

Index: test/mjsunit/concurrent-initial-prototype-change.js
diff --git a/test/mjsunit/concurrent-initial-prototype-change.js b/test/mjsunit/concurrent-initial-prototype-change.js
index 1b6f97b4332bbddc101d1f662ed5d6b5a3e900bf..ae02caf6a714ddc401d6e3014cb99d20732e77c9 100644
--- a/test/mjsunit/concurrent-initial-prototype-change.js
+++ b/test/mjsunit/concurrent-initial-prototype-change.js
@@ -27,6 +27,16 @@
 
 // Flags: --allow-natives-syntax
 // Flags: --concurrent-recompilation --block-concurrent-recompilation
+// Flags: --nostress-opt
+
+// --nostress-opt is in place because this particular optimization
+// (guaranteeing that the Array prototype chain has no elements) is
+// maintained isolate-wide. Once it's been "broken" by the change
+// to the Object prototype below, future compiles will not use the
+// optimization anymore, and the code will remain optimized despite
+// additional changes to the prototype chain.
+// TODO(mvstanton): A runtime function that clears this state could
+// allow us to run with --stress-opt.
 
 if (!%IsConcurrentRecompilationSupported()) {
   print("Concurrent recompilation is disabled. Skipping this test.");