Fix calculation of downstream caret location at the end of a string (#13426)

packages/flutter/lib/src/painting/text_painter.dart

@@ -416,7 +416,7 @@ class TextPainter {
   }
 
   Offset _getOffsetFromDownstream(int offset, Rect caretPrototype) {
-    final int nextCodeUnit = _text.codeUnitAt(offset + 1);
+    final int nextCodeUnit = _text.codeUnitAt(offset);
     if (nextCodeUnit == null)
       return null;
     final int nextRuneOffset = _isUtf16Surrogate(nextCodeUnit) ? offset + 2 : offset + 1;

packages/flutter/test/painting/text_painter_rtl_test.dart

@@ -287,7 +287,6 @@ void main() {
     expect( // between A and Alef, before the Alef
       painter.getOffsetForCaret(const TextPosition(offset: 1, affinity: TextAffinity.downstream), Rect.zero),
       const Offset(10.0, 10.0),
-      skip: skipExpectsWithKnownBugs, // this ends up on the wrong line currently - this is a major bug
     );
 
     expect( // after the Alef
@@ -318,7 +317,7 @@ void main() {
         const TextBox.fromLTRBD(0.0, 10.0, 10.0, 20.0, TextDirection.rtl), // Alef
       ],
     );
-  }, skip: skipTestsWithKnownBugs);
+  });
 
   test('TextPainter - line wrap mid-word', () {
     final TextPainter painter = new TextPainter()