Add a --harmony-object-observe runtime flag (on by default)

test/mjsunit/regress/regress-356589.js

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+// Flags: --harmony-object-observe
+
 // This test passes if it does not crash in debug mode
 
 arr = ['a', 'b', 'c', 'd'];
 Object.defineProperty(arr.__proto__, '0', { get: function(){} });
 Object.defineProperty(arr, '2', {get: function(){} });
 Object.observe(arr, function() {});
 arr.length = 2;