Plug TextDirection into box_decoration background gradient (#14810)

* Plug TextDirection into box_decoration background gradient

* Add test

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

@@ -325,7 +325,7 @@ class _BoxDecorationPainter extends BoxPainter {
 
   Paint _cachedBackgroundPaint;
   Rect _rectForCachedBackgroundPaint;
-  Paint _getBackgroundPaint(Rect rect) {
+  Paint _getBackgroundPaint(Rect rect, TextDirection textDirection) {
     assert(rect != null);
     assert(_decoration.gradient != null || _rectForCachedBackgroundPaint == null);
 
@@ -335,7 +335,7 @@ class _BoxDecorationPainter extends BoxPainter {
       if (_decoration.color != null)
         paint.color = _decoration.color;
       if (_decoration.gradient != null) {
-        paint.shader = _decoration.gradient.createShader(rect);
+        paint.shader = _decoration.gradient.createShader(rect, textDirection: textDirection);
         _rectForCachedBackgroundPaint = rect;
       }
       _cachedBackgroundPaint = paint;
@@ -376,7 +376,7 @@ class _BoxDecorationPainter extends BoxPainter {
 
   void _paintBackgroundColor(Canvas canvas, Rect rect, TextDirection textDirection) {
     if (_decoration.color != null || _decoration.gradient != null)
-      _paintBox(canvas, rect, _getBackgroundPaint(rect), textDirection);
+      _paintBox(canvas, rect, _getBackgroundPaint(rect, textDirection), textDirection);
   }
 
   DecorationImagePainter _imagePainter;

packages/flutter/test/painting/box_decoration_test.dart

@@ -42,4 +42,30 @@ void main() {
     expect(decoration.hitTest(size, const Offset(10.0, 10.0), textDirection: TextDirection.ltr), isFalse);
     expect(decoration.hitTest(size, const Offset(990.0, 10.0), textDirection: TextDirection.ltr), isTrue);
   });
+
+  test('BoxDecoration with LinearGradient using AlignmentDirectional', () {
+    const BoxDecoration decoration = const BoxDecoration(
+      color: const Color(0xFF000000),
+      gradient: const LinearGradient(
+        begin: AlignmentDirectional.centerStart,
+        end: AlignmentDirectional.bottomEnd,
+        colors: const<Color>[
+          const Color(0xFF000000),
+          const Color(0xFFFFFFFF),
+        ],
+      ),
+    );
+    final BoxPainter painter = decoration.createBoxPainter();
+    const Size size = const Size(1000.0, 1000.0);
+    expect(
+        (Canvas canvas) {
+        painter.paint(
+          canvas,
+          const Offset(0.0, 0.0),
+          const ImageConfiguration(size: size, textDirection: TextDirection.rtl),
+        );
+      },
+      paints..rect(rect: Offset.zero & size),
+    );
+  });
 }