Keep new arrays allocated with 'new Array(N)' in fast mode

test/mjsunit/es7/object-observe.js

 // Copyright 2012 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 1216 matching lines @@
   { object: arr3, type: 'splice', index: 1, removed: [], addedCount: 1 },
   { object: arr3, type: 'splice', index: 1, removed: [,], addedCount: 0 },
   { object: arr3, type: 'splice', index: 1, removed: [], addedCount: 4 },
   { object: arr3, name: '4', type: 'add' },
   { object: arr3, type: 'splice', index: 1, removed: [,,,5], addedCount: 0 }
 ]);
 
 
 // Updating length on large (slow) array
 reset();
-var slow_arr = new Array(1000000000);
+var slow_arr = %NormalizeElements([]);
 slow_arr[500000000] = 'hello';
+slow_arr.length = 1000000000;
 Object.observe(slow_arr, observer.callback);
 var spliceRecords;
 function slowSpliceCallback(records) {
   spliceRecords = records;
 }
 Array.observe(slow_arr, slowSpliceCallback);
 slow_arr.length = 100;
 Object.deliverChangeRecords(observer.callback);
 observer.assertCallbackRecords([
   { object: slow_arr, name: '500000000', type: 'delete', oldValue: 'hello' },
@@ skipping 539 matching lines @@
     for (var n1 = 0; n1 < 3; ++n1)
       for (var n2 = 0; n2 < 3; ++n2)
         for (var i in mutation)
           TestFastElementsLength(mutation[i], b1 != 0, b2 != 0, 20*n1, 20*n2);
 
 for (var b1 = 0; b1 < 2; ++b1)
   for (var b2 = 0; b2 < 2; ++b2)
     for (var n = 0; n < 3; ++n)
       for (var i in mutationByIncr)
         TestFastElementsLength(mutationByIncr[i], b1 != 0, b2 != 0, 7*n, 7*n+1);