Crash with backslash-escaped string in a place where a string is not expected

[srawlins]

Take this SCSS:

a {
  @extend '\A';
}

Ruby sass errors properly:

Error: Invalid CSS after "": expected selector, was "'\A'"
        on line 2 of d.scss

Dart sass crashes less gracefully:

Unexpected exception:
NoSuchMethodError: The method '>=' was called on null.
Receiver: null
Tried calling: >=(97)

dart:core                                                  Object.noSuchMethod
package:sass/src/util/character.dart 45:16                 isHex
package:sass/src/ast/sass/expression/string.dart 69:39     StringExpression.asInterpolation
package:sass/src/parse/stylesheet.dart 2145:56             StylesheetParser._almostAnyValue
package:sass/src/parse/stylesheet.dart 545:17              StylesheetParser._extendRule
package:sass/src/parse/stylesheet.dart 369:16              StylesheetParser._atRule
package:sass/src/parse/stylesheet.dart 87:16               StylesheetParser._statement
package:sass/src/parse/scss.dart 83:29                     ScssParser.children
package:sass/src/parse/stylesheet.dart 143:25              StylesheetParser._styleRule
package:sass/src/parse/stylesheet.dart 103:23              StylesheetParser._statement
package:sass/src/parse/stylesheet.dart 65:46               StylesheetParser.parse.<fn>.<fn>
package:sass/src/parse/scss.dart 121:32                    ScssParser.statements
package:sass/src/parse/stylesheet.dart 65:29               StylesheetParser.parse.<fn>
package:sass/src/parse/parser.dart 541:22                  Parser.wrapSpanFormatException
package:sass/src/parse/stylesheet.dart 63:12               StylesheetParser.parse
package:sass/src/ast/sass/statement/stylesheet.dart 40:56  Stylesheet.Stylesheet.parseScss
package:sass/sass.dart 31:13                               render
package:sass/src/executable.dart 50:15                     main