Can not specify toString/valueOf methods in object literal

TypeScript Version: 2.5.2

Code

interface X {
	name: string
}

const x1: X = {
	get name() { return 'x1' },
	toString() { return this.name },
//	^^^^^^^^
//	Type '{ readonly name: string; toString(): string; }' is not assignable to type 'X'.
//	Object literal may only specify known properties, and 'toString' does not exist in type 'X'.
}
const x2: X = {name: 'x2'}
x2.toString = function () { return this.name } // But add `toString` later is allowed.

Expected behavior:

Should allow object literal specify toString, valueOf and all properties exists on Object.prototype, or at least improve the error report to give a workaround (const x: X & Object).

Actual behavior:

See comments in code.

Start with the TypeScript 2.5.2 reproduction in the issue and compare object-literal excess-property checking for toString/valueOf with assigning the same method after initialization. Determine whether Object.prototype members should be accepted or whether the diagnostic should suggest the shown X & Object workaround. Done means the chosen behavior is implemented and covered by a regression test.