[streams] _flush not being called

In the following testcase the _flush callback is not being called after stream.end().

Simply removing the stream.write(null); line works fine.

Is it possible to end a stream with stream.write(null) and stream.end() or only the latter.

If so, why does stream.write(null) prevent stream.end() from triggering _flush?

var Transform = require('readable-stream/transform'),
    inherits  = require('util').inherits;

function MyTransform(opts){
  Transform.call(this, opts);
}

inherits(MyTransform, Transform);

MyTransform.prototype._transform = function( chunk, enc, next ){
  console.log('.');
  next();
};

MyTransform.prototype._flush = function(){
  console.log('END');
};

// ---

var stream = new MyTransform({ objectMode: true });

stream.write({ a: 'b' });
stream.write(null);
stream.end();

same behaviour on v0.11.14 and v0.10.33

ref: #89