[Gallery] Fix directory structure (#312)

gallery/lib/studies/shrine/supplemental/product_columns.dart

@@ -0,0 +1,80 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+import 'package:gallery/studies/shrine/model/product.dart';
+import 'package:gallery/studies/shrine/supplemental/product_card.dart';
+
+class TwoProductCardColumn extends StatelessWidget {
+  const TwoProductCardColumn({
+    @required this.bottom,
+    this.top,
+    @required this.imageAspectRatio,
+  }) : assert(bottom != null);
+
+  static const double spacerHeight = 44;
+  static const double horizontalPadding = 28;
+
+  final Product bottom, top;
+  final double imageAspectRatio;
+
+  @override
+  Widget build(BuildContext context) {
+    return LayoutBuilder(builder: (context, constraints) {
+      return ListView(
+        physics: const ClampingScrollPhysics(),
+        children: [
+          Padding(
+            padding: const EdgeInsetsDirectional.only(start: horizontalPadding),
+            child: top != null
+                ? MobileProductCard(
+                    imageAspectRatio: imageAspectRatio,
+                    product: top,
+                  )
+                : SizedBox(
+                    height: spacerHeight,
+                  ),
+          ),
+          const SizedBox(height: spacerHeight),
+          Padding(
+            padding: const EdgeInsetsDirectional.only(end: horizontalPadding),
+            child: MobileProductCard(
+              imageAspectRatio: imageAspectRatio,
+              product: bottom,
+            ),
+          ),
+        ],
+      );
+    });
+  }
+}
+
+class OneProductCardColumn extends StatelessWidget {
+  const OneProductCardColumn({
+    this.product,
+    @required this.reverse,
+  });
+
+  final Product product;
+
+  // Whether the product column should align to the bottom.
+  final bool reverse;
+
+  @override
+  Widget build(BuildContext context) {
+    return ListView(
+      physics: const ClampingScrollPhysics(),
+      reverse: reverse,
+      children: [
+        const SizedBox(
+          height: 40,
+        ),
+        MobileProductCard(
+          product: product,
+        ),
+      ],
+    );
+  }
+}