Comments not preserved in referenced includes

[betaorbust]

Problem

When a file is included with (reference) in-property comments are stripped out. This breaks the only workaround to #2674.

Bug example

Input

// test.less
@import (reference) 'imported-file.less';
.my-test{
    .my-mixin();
}

// imported-file.less
.my-mixin(){
    color: red /* comment that should be there */;
}

Output

.my-test {
  color: red;
}

Expected Output

.my-test {
  color: red /* comment that should be there */;
}