Add a golden image test for centered text (#17517)

bin/internal/goldens.version

-ed07132380cd2db6dd664be1479c5b89579e3304
+df3f713cc47fc4e919d1c5cf52c0543424c26c09

packages/flutter/test/widgets/text_golden_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io' show Platform;
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('Centered text', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new RepaintBoundary(
+        child: new Center(
+          child: new Container(
+            width: 200.0,
+            height: 100.0,
+            decoration: const BoxDecoration(
+              color: const Color(0xff00ff00),
+            ),
+            child: const Text('Hello',
+              textDirection: TextDirection.ltr,
+              textAlign: TextAlign.center,
+              style: const TextStyle(color: const Color(0xffff0000)),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await expectLater(
+      find.byType(Container),
+      matchesGoldenFile('text_golden.Centered.png'),
+      skip: !Platform.isLinux,
+    );
+
+    await tester.pumpWidget(
+      new RepaintBoundary(
+        child: new Center(
+          child: new Container(
+            width: 200.0,
+            height: 100.0,
+            decoration: const BoxDecoration(
+              color: const Color(0xff00ff00),
+            ),
+            child: const Text('Hello world how are you today',
+              textDirection: TextDirection.ltr,
+              textAlign: TextAlign.center,
+              style: const TextStyle(color: const Color(0xffff0000)),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await expectLater(
+      find.byType(Container),
+      matchesGoldenFile('text_golden.Centered.wrap.png'),
+      skip: !Platform.isLinux,
+    );
+  });
+}