Including a @mixin when outside of selector still outputs properties

[ezekg]

As mentioned in ezekg/flint#40 (comment) and similar to #1585.

Reproducable test case:

@mixin a {
  b: c;
}

@include a();

Compiling with Ruby Sass results in:

Error: Properties are only allowed within rules, directives, mixin includes, or other properties.

Cmd: sass test.scss

Compiling with Libsass results in:

b: c;

Cmd: sassc test.scss

[xzyfer]

Thanks for the report @ezekg.

[xzyfer]

Spec added sass/sass-spec#613

[mgreter]

This is a bit more complicated since the check cannot be done in parser, since the following is valid:

@mixin a {
  b: c;
}
foo {
  @include a();
}

So we need to do this check after the mixin is expanded.

[mgreter]

WIP to fix this in #1900

[xzyfer]

Reassiging this to 3.4. This doesn't prevent people to from migrating to LibSass I suspect browsers will simply ignore these erroneous properties.