Layout row app + tests

Contributes to #2233

Author: chalin

examples/layout/row/README.md

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+../../README.md

examples/layout/row/images/pic1.jpg

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+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart' show debugPaintSizeEnabled;
+
+void main() {
+  debugPaintSizeEnabled = true; // Remove to suppress visual layout
+  runApp(MyApp());
+}
+
+// #docregion MyApp
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter layout demo',
+      home: MyHomePage(title: 'Flutter layout demo'),
+    );
+  }
+}
+
+class MyHomePage extends StatefulWidget {
+  MyHomePage({Key key, this.title}) : super(key: key);
+
+  final String title;
+
+  @override
+  _MyHomePageState createState() => _MyHomePageState();
+}
+
+class _MyHomePageState extends State<MyHomePage> {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(widget.title),
+      ),
+      body: Center(
+        child: Row(
+          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+          children: [
+            Image.asset('images/pic1.jpg'),
+            Image.asset('images/pic2.jpg'),
+            Image.asset('images/pic3.jpg'),
+          ],
+        ),
+      ),
+    );
+  }
+}

examples/layout/row/images/pic2.jpg

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+name: layout
+description: >
+  Sample app from "Building Layouts", https://flutter.io/docs/development/ui/layout.
+version: 1.0.0
+
+environment:
+  sdk: '>=2.0.0-dev.68.0 <3.0.0'
+
+dependencies:
+  flutter:
+    sdk: flutter
+  cupertino_icons: ^0.1.2
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
+flutter:
+  uses-material-design: true
+  assets:
+    - images/pic1.jpg
+    - images/pic2.jpg
+    - images/pic3.jpg

examples/layout/row/images/pic3.jpg

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+// This is a basic Flutter widget test.
+//
+// To perform an interaction with a widget in your test, use the WidgetTester
+// utility that Flutter provides. For example, you can send tap and scroll
+// gestures. You can also use WidgetTester to find child widgets in the widget
+// tree, read text, and verify that the values of widget properties are correct.
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:layout/main.dart';
+
+void main() {
+  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(MyApp());
+
+    expect(find.text('Flutter layout demo'), findsOneWidget);
+  });
+}