anchor.diff:        (character >= $a && character <= $f) ||
         (character >= $A && character <= $F);
   
  +  bool _isExpressionStart(int character) =>
  +      character == $lparen || character == $slash || character == $dot ||
  +      character == $lbracket || character == $single_quote ||
  +      character == $double_quote || character == $hash || character == $plus ||
  +      character == $minus || character == $backslash ||
  +      _isNameStart(character) || _isDigit(character);
  +
     int _asHex(int character) {
       if (character <= $9) return character - $0;
       if (character <= $F) return 10 + character - $A;
  
positive.code: 
    // ## Utilities
  
    /// Based on [the CSS algorithm][], but also considers interpolation to be
    /// valid in an identifier.
    ///
    /// [the CSS algorithm]: https://drafts.csswg.org/css-syntax-3/#would-start-an-identifier
    bool _lookingAtInterpolatedIdentifier() {
      var first = _scanner.peekChar();
      if (_isNameStart(first)) return true;
      if (first == $backslash) return !_isNewline(_scanner.peekChar(1));
      if (first == $hash) return _scanner.peekChar(1) == $lbrace;
  
      if (first != $dash) return false;
  
      var second = _scanner.peekChar();
      if (_isNameStart(second)) return true;
      if (second == $hash) return _scanner.peekChar(2) == $lbrace;
      return second == $backslash && !_isNewline(_scanner.peekChar(2));
    }
  
    String _rawText(void consumer()) {
  
negative.code:       _ignoreComments();
      }
  
      if (commaExpressions.isEmpty) _scanner.error("Expected expression.");
      if (!hadComma) return commaExpressions.single;
      return new ListExpression(commaExpressions, ListSeparator.comma,
          span: _scanner.spanFrom(start));
    }
  
    Expression _singleExpression() {
      var first = _scanner.peekChar();
      switch (first) {
        // Note: when adding a new case, make sure it's reflected in
        // [_isExpressionStart].
        case $lparen: return _parentheses();
        case $slash: return _unaryOperator();
        case $dot: return _number();
  