Protect the emptiness of Array prototype elements with a PropertyCell.

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

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 // 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.
 
 if (!%IsConcurrentRecompilationSupported()) {
   print("Concurrent recompilation is disabled. Skipping this test.");
   quit();
 }
 
 function f1(a, i) {
   return a[i] + 0.5;
 }
 
@@ skipping 10 matching lines @@
 assertEquals(2, f1(arr, 1));
 // Not yet optimized since concurrent recompilation is blocked.
 assertUnoptimized(f1, "no sync");
 // Let concurrent recompilation proceed.
 %UnblockConcurrentRecompilation();
 // Sync with background thread to conclude optimization, which bails out
 // due to map dependency.
 assertUnoptimized(f1, "sync");
 //Clear type info for stress runs.
 %ClearFunctionTypeFeedback(f1);