HttpFoundation Request POST parameters
Apparently with Vue Resource, I need to add emulateJSON:


  emulateJSON boolean Send request body as application/x-www-form-urlencoded content type.


this.$http.post('http://mydrupal8.url/simple_path', {
  params: {
    'message': 'hello world'
  },
  headers: {
    'Access-Control-Allow-Origin': 'http://localhost:8080',
  }
}, {
  emulateJSON: true
}).then(response =&gt; {

  console.log(response);

}, response =&gt; {
  // error callback
});


Now in my D8 backend:

$params = $request-&gt;request-&gt;get('params');
$message = $params['message'];


And now it works as expected.