Writable's instanceof implementation is faulty #14943

• Version: Node v8.4.0
• Platform: macOS
• Subsystem: Stream

The implementation for Writable's @@hasInstance method is faulty. As described in this Stack Overflow post:

In this snippet, the statement f instanceof PipeWritable returns true (Node v8.4.0):

const stream = require('stream');
const fs = require('fs');

class PipeWritable extends stream.Writable {
    constructor () {
        super();
    }
}

const s = new PipeWritable();
const f = fs.createWriteStream('/tmp/test');

console.log(f instanceof PipeWritable); // true ... ???

Because of the hasInstance implementation, this line:

Allows for Writables to be subclasses of Writable. This is a side-effect of Duplex parasitically inheriting from Writable, making new Duplex() instanceof Writeable true but also making Writables instances of Writable subclasses. Maybe add another check to make sure only Duplexes can be instances of Writables, but not Writables instances of Writable subclasses?

_{[refack: replaced code copy with code reference]}