Add Opacity unit test

@@ -36,6 +36,7 @@ import 'widget_container_test.dart' as widget_container;
 import 'widget_flex_test.dart' as widget_flex;
 import 'widget_grid_view_test.dart' as widget_grid_view;
 import 'widget_multipage_test.dart' as widget_multipage;
+import 'widget_opacity_test.dart' as widget_opacity;
 import 'widget_partitions_test.dart' as widget_partitions;
 import 'widget_table_test.dart' as widget_table;
 import 'widget_test.dart' as widget;
@@ -65,6 +66,7 @@ void main() {
   widget_flex.main();
   widget_grid_view.main();
   widget_multipage.main();
+  widget_opacity.main();
   widget_partitions.main();
   widget_table.main();
   widget_text.main();

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * 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.
+ */
+
+// ignore_for_file: omit_local_variable_types
+
+import 'dart:io';
+
+import 'package:pdf/widgets.dart';
+import 'package:test/test.dart';
+
+Document pdf;
+
+void main() {
+  setUpAll(() {
+    Document.debug = true;
+    pdf = Document();
+  });
+
+  test('Opacity Widgets', () {
+    pdf.addPage(
+      Page(
+        build: (Context context) => Stack(
+          alignment: Alignment.center,
+          children: <Widget>[
+            Text('Background', textScaleFactor: 5),
+            Opacity(
+              opacity: .5,
+              child: PdfLogo(),
+            ),
+          ],
+        ),
+      ),
+    );
+  });
+
+  tearDownAll(() {
+    final File file = File('widgets-opacity.pdf');
+    file.writeAsBytesSync(pdf.save());
+  });
+}