Variables placed outside a mixin are not properly compiled

[tarekoraby]

In Vaadin 8.13.0.beta1, variables that are used outside a mixin do not get properly compiled.

To reproduce, compile the following:

@import "../valo/valo.scss";

$custom-value: 500px;

@media only screen and (min-width: $custom-value) {
  .my-style {
    color: red;
    background-color: lightblue;
  }
}

@mixin mytheme {
  @include valo;

  // Insert your own theme rules here
}

The expected behavior is that the CSS should have $custom-value translated to 500px.
However, in reality, the CSS includes a faulty $custom-value)