Preserve transform when using *Gradient:withOpacity (flutter#154908)

i don't think it was intentional to lose it?

- [] All existing and new tests are passing.

Author: hyhugh

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

@@ -570,6 +570,7 @@ class LinearGradient extends Gradient {
       ],
       stops: stops,
       tileMode: tileMode,
+      transform: transform,
     );
   }
 }
@@ -872,6 +873,7 @@ class RadialGradient extends Gradient {
       tileMode: tileMode,
       focal: focal,
       focalRadius: focalRadius,
+      transform: transform,
     );
   }
 }
@@ -1149,6 +1151,7 @@ class SweepGradient extends Gradient {
       ],
       stops: stops,
       tileMode: tileMode,
+      transform: transform,
     );
   }
 }

packages/flutter/test/painting/gradient_test.dart

@@ -445,7 +445,32 @@ void main() {
         Color(0x80777777),
         Color(0x80444444),
       ],
-    ),);
+    ));
+  });
+
+  test('LinearGradient withOpacity() preserves transform', () {
+    const LinearGradient testGradient = LinearGradient(
+      begin: Alignment.bottomRight,
+      end: Alignment.topCenter,
+      colors: <Color>[
+        Color(0xFFFFFFFF),
+        Color(0xAF777777),
+        Color(0x44444444),
+      ],
+      transform: GradientRotation(1),
+    );
+    final LinearGradient actual = testGradient.withOpacity(0.5);
+
+    expect(actual, const LinearGradient(
+      begin: Alignment.bottomRight,
+      end: Alignment.topCenter,
+      colors: <Color>[
+        Color(0x80FFFFFF),
+        Color(0x80777777),
+        Color(0x80444444),
+      ],
+      transform: GradientRotation(1),
+    ));
   });
 
   test('RadialGradient with AlignmentDirectional', () {
@@ -765,6 +790,36 @@ void main() {
       ],
     ));
   });
+
+  test('RadialGradient withOpacity() preserves transform', () {
+    const RadialGradient testGradient = RadialGradient(
+      center: Alignment.topLeft,
+      focal: Alignment.centerLeft,
+      radius: 20.0,
+      focalRadius: 10.0,
+      colors: <Color>[
+        Color(0xFFFFFFFF),
+        Color(0xAF777777),
+        Color(0x44444444),
+      ],
+      transform: GradientRotation(1),
+    );
+    final RadialGradient actual = testGradient.withOpacity(0.5);
+
+    expect(actual, const RadialGradient(
+      center: Alignment.topLeft,
+      focal: Alignment.centerLeft,
+      radius: 20.0,
+      focalRadius: 10.0,
+      colors: <Color>[
+        Color(0x80FFFFFF),
+        Color(0x80777777),
+        Color(0x80444444),
+      ],
+      transform: GradientRotation(1),
+    ));
+  });
+
   test('SweepGradient lerp test', () {
     const SweepGradient testGradient1 = SweepGradient(
       center: Alignment.topLeft,
@@ -997,6 +1052,32 @@ void main() {
       ],
     ));
   });
+
+  test('SweepGradient withOpacity() preserves transform', () {
+    const SweepGradient testGradient = SweepGradient(
+      center: Alignment.topLeft,
+      endAngle: math.pi / 2,
+      colors: <Color>[
+        Color(0xFFFFFFFF),
+        Color(0xAF777777),
+        Color(0x44444444),
+      ],
+      transform: GradientRotation(1),
+    );
+    final SweepGradient actual = testGradient.withOpacity(0.5);
+
+    expect(actual, const SweepGradient(
+      center: Alignment.topLeft,
+      endAngle: math.pi / 2,
+      colors: <Color>[
+        Color(0x80FFFFFF),
+        Color(0x80777777),
+        Color(0x80444444),
+      ],
+      transform: GradientRotation(1),
+    ));
+  });
+
   test('Gradient lerp test (with RadialGradient)', () {
     const RadialGradient testGradient1 = RadialGradient(
       center: Alignment.topLeft,