Fix bug in JSON.parse for objects containing "__proto__" as key.

test/mjsunit/json.js

Index: test/mjsunit/json.js
diff --git a/test/mjsunit/json.js b/test/mjsunit/json.js
index a0be8dd133504f192403335d4523445b52b332d8..812ffeb5a7ba3c74ca51f4cec34ee32945a88686 100644
--- a/test/mjsunit/json.js
+++ b/test/mjsunit/json.js
@@ -415,3 +415,17 @@ var falseNum = Object("37");
 falseNum.__proto__ = Number.prototype;
 falseNum.toString = function() { return 42; };
 assertEquals('"42"', JSON.stringify(falseNum));
+
+// We don't currently allow plain properties called __proto__ in JSON
+// objects in JSON.parse. Instead we read them as we would JS object
+// literals. If we change that, this test should change with it.
+// 
+// Parse a non-object value as __proto__. This must not create a 
+// __proto__ property different from the original, and should not 
+// change the original.
+var o = JSON.parse('{"__proto__":5}');
+assertEquals(Object.prototype, o.__proto__);  // __proto__ isn't changed.
+assertEquals(0, Object.keys(o).length);  // __proto__ isn't added as enumerable.
+
+
+