Adding subtitle to ExpansionTile (#43946)

* Adding subtitle to expansion_tile

packages/flutter/lib/src/material/expansion_tile.dart

@@ -35,6 +35,7 @@ class ExpansionTile extends StatefulWidget {
     Key key,
     this.leading,
     @required this.title,
+    this.subtitle,
     this.backgroundColor,
     this.onExpansionChanged,
     this.children = const <Widget>[],
@@ -53,6 +54,11 @@ class ExpansionTile extends StatefulWidget {
   /// Typically a [Text] widget.
   final Widget title;
 
+  /// Additional content displayed below the title.
+  ///
+  /// Typically a [Text] widget.
+  final Widget subtitle;
+
   /// Called when the tile expands or collapses.
   ///
   /// When the tile starts expanding, this function is called with the value
@@ -161,6 +167,7 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
               onTap: _handleTap,
               leading: widget.leading,
               title: widget.title,
+              subtitle: widget.subtitle,
               trailing: widget.trailing ?? RotationTransition(
                 turns: _iconTurns,
                 child: const Icon(Icons.expand_more),

packages/flutter/test/material/expansion_tile_test.dart

@@ -215,4 +215,20 @@ void main() {
     expect(iconColor(expandedIconKey), _unselectedWidgetColor);
     expect(iconColor(collapsedIconKey), _accentColor);
   });
+
+  testWidgets('ExpansionTile subtitle', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: ExpansionTile(
+            title: Text('Title'),
+            subtitle: Text('Subtitle'),
+            children: <Widget>[ListTile(title: Text('0'))],
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('Subtitle'), findsOneWidget);
+  });
 }