Does not work with the passthrough option of Gulp 4

``` javascript
var gulp = require('gulp');
var concat = require('gulp-concat');

gulp.task(function foo() {
  return gulp.src('src/*.css', {
    passthrough: true,
  })
    .pipe(concat('styles.css'))
    .pipe(gulp.dest('dist/'))
  ;
});
```

`styles.css` is never written.

It does work with [gulp-continuous-concat](https://github.com/Kinvey/gulp-continuous-concat).
