Add tests for Baseline. (#8021)

packages/flutter/test/widgets/baseline_test.dart

+// Copyright 2016 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 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('Baseline - control test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new DefaultTextStyle(
+          style: new TextStyle(
+            fontFamily: 'Ahem',
+            fontSize: 100.0,
+          ),
+          child: new Text('X'),
+        ),
+      ),
+    );
+    expect(tester.renderObject<RenderBox>(find.text('X')).size, const Size(100.0, 100.0));
+  });
+
+  testWidgets('Baseline - position test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new Baseline(
+          baseline: 180.0,
+          baselineType: TextBaseline.alphabetic,
+          child: new DefaultTextStyle(
+            style: new TextStyle(
+              fontFamily: 'Ahem',
+              fontSize: 100.0,
+            ),
+            child: new Text('X'),
+          ),
+        ),
+      ),
+    );
+    expect(tester.renderObject<RenderBox>(find.text('X')).size, const Size(100.0, 100.0));
+    expect(tester.renderObject<RenderBox>(find.byType(Baseline)).size, const Size(100.0, 200.0));
+  });
+}