ES6: Remove __proto__ setter poison pill

test/mjsunit/proto-poison.js

-// Copyright 2013 the V8 project authors. All rights reserved.
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-
-// Check that the __proto__ accessor is properly poisoned when extracted
-// from Object.prototype using the property descriptor.
-var desc = Object.getOwnPropertyDescriptor(Object.prototype, "__proto__");
-assertEquals("function", typeof desc.get);
-assertEquals("function", typeof desc.set);
-assertDoesNotThrow("desc.get.call({})");
-assertThrows("desc.set.call({})", TypeError);
-
-// Check that any redefinition of the __proto__ accessor causes poising
-// to cease and the accessor to be extracted normally.
-Object.defineProperty(Object.prototype, "__proto__", { get:function(){} });
-desc = Object.getOwnPropertyDescriptor(Object.prototype, "__proto__");
-assertDoesNotThrow("desc.get.call({})");
-assertThrows("desc.set.call({})", TypeError);
-Object.defineProperty(Object.prototype, "__proto__", { set:function(x){} });
-desc = Object.getOwnPropertyDescriptor(Object.prototype, "__proto__");
-assertDoesNotThrow("desc.get.call({})");
-assertDoesNotThrow("desc.set.call({})");