Convert TextSpan usage to new InlineSpan API

diff --git a/packages/flutter_markdown/lib/src/builder.dart b/packages/flutter_markdown/lib/src/builder.dart
index 1befc97..4a912b7 100644
--- a/packages/flutter_markdown/lib/src/builder.dart
+++ b/packages/flutter_markdown/lib/src/builder.dart
@@ -340,9 +340,10 @@
     for (Widget child in inline.children) {
       if (mergedTexts.isNotEmpty && mergedTexts.last is RichText && child is RichText) {
         RichText previous = mergedTexts.removeLast();
-        List<TextSpan> children = previous.text.children != null
-            ? new List.from(previous.text.children)
-            : [previous.text];
+        TextSpan previousTextSpan = previous.text;
+        List<TextSpan> children = previousTextSpan.children != null
+            ? new List.from(previousTextSpan.children)
+            : [previousTextSpan];
         children.add(child.text);
         TextSpan mergedSpan = new TextSpan(children: children);
         mergedTexts.add(new RichText(
diff --git a/packages/flutter_markdown/test/flutter_markdown_test.dart b/packages/flutter_markdown/test/flutter_markdown_test.dart
index 3e833fe..3949800 100644
--- a/packages/flutter_markdown/test/flutter_markdown_test.dart
+++ b/packages/flutter_markdown/test/flutter_markdown_test.dart
@@ -183,17 +183,19 @@
 
       final RichText firstTextWidget =
           tester.allWidgets.firstWhere((Widget widget) => widget is RichText);
+      final TextSpan firstTextSpan = firstTextWidget.text;
       final Image image =
           tester.allWidgets.firstWhere((Widget widget) => widget is Image);
       final NetworkImage networkImage = image.image;
       final RichText secondTextWidget =
           tester.allWidgets.lastWhere((Widget widget) => widget is RichText);
+      final TextSpan secondTextSpan = secondTextWidget.text;
 
-      expect(firstTextWidget.text.text, 'textbefore ');
-      expect(firstTextWidget.text.style.fontStyle, FontStyle.italic);
+      expect(firstTextSpan.text, 'textbefore ');
+      expect(firstTextSpan.style.fontStyle, FontStyle.italic);
       expect(networkImage.url,'http://img');
-      expect(secondTextWidget.text.text, ' textafter');
-      expect(secondTextWidget.text.style.fontStyle, FontStyle.italic);
+      expect(secondTextSpan.text, ' textafter');
+      expect(secondTextSpan.style.fontStyle, FontStyle.italic);
     });
 
     testWidgets('should work with a link', (WidgetTester tester) async {