Adding shrine. (#3)

Adds a new version of Shrine from the MDC codelabs.

shrine/lib/supplemental/product_columns.dart

@@ -0,0 +1,87 @@
+// Copyright 2018-present the Flutter authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'package:flutter/material.dart';
+
+import '../model/product.dart';
+import 'product_card.dart';
+
+class TwoProductCardColumn extends StatelessWidget {
+  TwoProductCardColumn({
+    this.bottom,
+    this.top,
+  }) : assert(bottom != null);
+
+  final Product bottom, top;
+
+  @override
+  Widget build(BuildContext context) {
+    return LayoutBuilder(
+        builder: (BuildContext context, BoxConstraints constraints) {
+      const spacerHeight = 44.0;
+
+      double heightOfCards = (constraints.biggest.height - spacerHeight) / 2.0;
+      double heightOfImages = heightOfCards - ProductCard.kTextBoxHeight;
+      double imageAspectRatio =
+          (heightOfImages >= 0.0 && constraints.biggest.width > heightOfImages)
+              ? constraints.biggest.width / heightOfImages
+              : 33 / 49;
+
+      return ListView(
+        children: <Widget>[
+          Padding(
+            padding: EdgeInsetsDirectional.only(start: 28.0),
+            child: top != null
+                ? ProductCard(
+                    imageAspectRatio: imageAspectRatio,
+                    product: top,
+                  )
+                : SizedBox(
+                    height: heightOfCards,
+                  ),
+          ),
+          SizedBox(height: spacerHeight),
+          Padding(
+            padding: EdgeInsetsDirectional.only(end: 28.0),
+            child: ProductCard(
+              imageAspectRatio: imageAspectRatio,
+              product: bottom,
+            ),
+          ),
+        ],
+      );
+    });
+  }
+}
+
+class OneProductCardColumn extends StatelessWidget {
+  OneProductCardColumn({this.product});
+
+  final Product product;
+
+  @override
+  Widget build(BuildContext context) {
+    return ListView(
+      reverse: true,
+      children: <Widget>[
+        SizedBox(
+          height: 40.0,
+        ),
+        ProductCard(
+          product: product,
+        ),
+      ],
+    );
+  }
+}