title: Flutter widget unittests throws No Directionality widget found
body: <p>i try to unittest a small own widget:</p>
  
  <pre><code>testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    var text = "abc";
    var label = "def";
  
    await tester.pumpWidget(LabeledTextWidget(
      text,
      label: label,
    ));
  
    final textFinder = find.text(text);
    final labelFinder = find.text(label);
  
    expect(textFinder , findsOneWidget);
    expect(labelFinder, findsOneWidget);
  });
  </code></pre>
  
  <p>Here is the widget code:</p>
  
  <pre><code>class LabeledTextWidget extends StatelessWidget {
    final String text;
    final String label;
    LabeledTextWidget(this.text, {this.label});
  
    @override
    Widget build(BuildContext context) {
      return Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
          children: &lt;Widget&gt;[
           Text(label,style: Theme.of(context).textTheme.caption,),
           Text(text),
           ],
      );
    }
  </code></pre>
  
  <p>The test always throws the exception:</p>
  
  <pre><code>...
  No Directionality widget found.
  RichText widgets require a Directionality widget ancestor.
  The specific widget that could not find a Directionality ancestor was:
  RichText
  ...
  </code></pre>
  
  <p>I can avoid this error by adding a text direction to all Text-Widgets. (textDirection: TextDirection.ltr,), but i thats a bad solution and it doesn´t work with rows.</p>