Symbol.valueOf method returns a symbol type

http://www.ecma-international.org/ecma-262/6.0/#sec-symbol.prototype.valueof

**TypeScript Version:**  master

**Code**

```diff
interface Symbol {
    /** Returns a string representation of an object. */
    toString(): string;

    /** Returns the primitive value of the specified object. */
-    valueOf(): Object;
+    valueOf(): symbol;
}
```