Dart Sass 1.0.0-alpha.7

• Fix function-exists(), variable-exists(), and mixin-exists() to use the lexical scope rather than always using the global scope.
• str-index() now correctly inserts at negative indices.
• Properly parse url()s that contain comment-like text.
• Fix a few more small @extend bugs.
• Fix a bug where interpolation in a quoted string was being dropped in some circumstances.
• Properly handle @for rules where each bound has a different unit.
• Forbid mixins and functions from being defined in control directives.
• Fix a superselector-computation edge case involving :not().
• Gracefully handle input files that are invalid UTF-8.
• Print a Sass stack trace when a file fails to load.