Fix str-slice() with astral plane characters

Closes #732

CHANGELOG.md

@@ -22,6 +22,10 @@
 * The `alpha()` function now produces clearer error messages when the wrong
   number of arguments are passed.
 
+* Fix a bug where the `str-slice()` function could produce invalid output when
+  passed a string that contains characters that aren't represented as a single
+  byte in UTF-16.
+
 ## 1.22.2
 
 ### JavaScript API

lib/src/functions/string.dart

@@ -113,7 +113,7 @@ final _slice =
   return SassString(
       string.text.substring(
           codepointIndexToCodeUnitIndex(string.text, startCodepoint),
-          codepointIndexToCodeUnitIndex(string.text, endCodepoint) + 1),
+          codepointIndexToCodeUnitIndex(string.text, endCodepoint + 1)),
       quotes: string.hasQuotes);
 });