Add a test component with a golden test

lib/components/my_component.dart

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+import 'package:flutter/material.dart';
+
+class MyComponent extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      mainAxisAlignment: MainAxisAlignment.center,
+      children: [
+        Text('This is my text'),
+        Text('This is another text'),
+        RaisedButton(
+          onPressed: () {},
+          child: Text('This should be a button'),
+          color: Colors.amber,
+        ),
+        Switch(
+          onChanged: (_) {},
+          value: true,
+        ),
+        Slider(
+          value: 2.0,
+          max: 7.0,
+          onChanged: (_) {},
+        ),
+      ],
+    );
+  }
+}

test/components/my_component_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+
+import 'package:showcase/components/my_component.dart';
+import '../helpers/golden_boundary.dart';
+
+void main() {
+  testWidgets('MyComponent golden', (WidgetTester tester) async {
+    await tester.pumpWidget(GoldenBoundary(
+      child: MyComponent(),
+    ));
+
+    await expectLater(
+      find.byType(Container).first,
+      matchesGoldenFile('golden.png'),
+    );
+  });
+}

test/helpers/golden_boundary.dart

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+import 'package:flutter/material.dart';
+
+class GoldenBoundary extends StatelessWidget {
+  final Widget child;
+
+  GoldenBoundary({@required this.child});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        body: RepaintBoundary(
+          child: Container(
+            padding: EdgeInsets.all(10.0),
+            width: 640.0,
+            height: 480.0,
+            child: child,
+          ),
+        ),
+      ),
+    );
+  }
+}