Fix a bug in for/in iteration of arguments objects.

test/mjsunit/regress/regress-1531.js

+// Copyright 2011 the V8 project authors. All rights reserved.
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+// modification, are permitted provided that the following conditions are
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+
+// Regression test for computing elements keys of arguments object.  Should
+// not crash or assert.
+function test(x) {
+  arguments[10] = 0;
+  var arr = [];
+  for (var p in arguments) arr.push(p);
+  return arr;
+}
+assertEquals(["0", "10"], test(0));
+
+// Regression test for lookup after delete of a dictionary-mode arguments
+// backing store.  Should not crash or assert.
+function test1(x, y, z) {
+  // Put into dictionary mode.
+  arguments.__defineSetter__("5", function () { return 0; });
+  // Delete a property from the dictionary.
+  delete arguments[5];
+  // Look up a property in the dictionary.
+  return arguments[2];
+}
+
+assertEquals(void 0, test1(0));