Apply the paint offset to the bounds rectangle for shader masks (#10458)

Fixes https://github.com/flutter/flutter/issues/10424

packages/flutter/lib/src/rendering/object.dart

@@ -416,7 +416,7 @@ class PaintingContext {
     _stopRecordingIfNeeded();
     final ShaderMaskLayer shaderLayer = new ShaderMaskLayer(
       shader: shader,
-      maskRect: maskRect,
+      maskRect: maskRect.shift(offset),
       blendMode: blendMode,
     );
     _appendLayer(shaderLayer);

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -834,8 +834,8 @@ class RenderShaderMask extends RenderProxyBox {
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
       assert(needsCompositing);
-      final Rect rect = Offset.zero & size;
+      final Shader shader = _shaderCallback(offset & size);
-      context.pushShaderMask(offset, _shaderCallback(rect), rect, _blendMode, super.paint);
+      context.pushShaderMask(offset, shader, Offset.zero & size, _blendMode, super.paint);
     }
   }
 }

packages/flutter/test/widgets/shader_mask_test.dart

@@ -22,4 +22,28 @@ void main() {
     final Widget child = new Container(width: 100.0, height: 100.0);
     await tester.pumpWidget(new ShaderMask(child: child, shaderCallback: createShader));
   });
+
+  testWidgets('Bounds rect includes offset', (WidgetTester tester) async {
+    Rect shaderBounds;
+    Shader recordShaderBounds(Rect bounds) {
+      shaderBounds = bounds;
+      return createShader(bounds);
+    }
+
+    final Widget widget = new Align(
+      alignment: FractionalOffset.center,
+      child: new SizedBox(
+        width: 400.0,
+        height: 400.0,
+        child: new ShaderMask(
+          shaderCallback: recordShaderBounds,
+          child: new Container(width: 100.0, height: 100.0)
+        ),
+      ),
+    );
+    await tester.pumpWidget(widget);
+
+    // The shader bounds rectangle should reflect the position of the centered SizedBox.
+    expect(shaderBounds, equals(new Rect.fromLTWH(200.0, 100.0, 400.0, 400.0)));
+  });
 }