Incorrectly interprets native CSS mixins as nested rules

[tony19]

See sass/sass#2235

input.scss

$color: #f00;
my-view1 {
  --view1-card: {
    border: solid 1px #{$color};
    opacity: 0.5;
  };
}

libsass 3.5.0.beta.2

my-view1 {
  --view1-card-border: solid 1px #f00;
  --view1-card-opacity: 0.5;
}

ruby sass 3.4.21

my-view1 {
  --view1-card-border: solid 1px #f00;
  --view1-card-opacity: 0.5;
}

version info:

$ node-sass --version
node-sass	4.4.0	(Wrapper)	[JavaScript]
libsass  	3.5.0.beta.2	(Sass Compiler)	[C/C++]

[xzyfer]

I believe this is a duplicate of #2076