Trailing widget override for ExpansionTile (#11904)

* Trailing widget override for ExpansionTile

Fixes #11890

* Add a test

* Replaced missing comma

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

@@ -38,6 +38,7 @@ class ExpansionTile extends StatefulWidget {
     this.backgroundColor,
     this.onExpansionChanged,
     this.children: const <Widget>[],
+    this.trailing,
   }) : super(key: key);
 
   /// A widget to display before the title.
@@ -64,6 +65,9 @@ class ExpansionTile extends StatefulWidget {
 
   /// The color to display behind the sublist when expanded.
   final Color backgroundColor;
+  
+  /// A widget to display instead of a rotating arrow icon.
+  final Widget trailing;
 
   @override
   _ExpansionTileState createState() => new _ExpansionTileState();
@@ -145,7 +149,7 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
                 style: Theme.of(context).textTheme.subhead.copyWith(color: titleColor),
                 child: widget.title,
               ),
-              trailing: new RotationTransition(
+              trailing: widget.trailing ?? new RotationTransition(
                 turns: _iconTurns,
                 child: const Icon(Icons.expand_more),
               ),

packages/flutter/test/material/two_level_list_test.dart

@@ -103,4 +103,33 @@ void main() {
     await tester.tap(find.text('Sublist'));
     expect(didChangeOpen, isFalse);
   });
+
+  testWidgets('trailing override', (WidgetTester tester) async {
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+      '/': (_) {
+        return new Material(
+          child: new SingleChildScrollView(
+            child: new Column(
+              children: <Widget>[
+                const ListTile(title: const Text('Top')),
+                new ExpansionTile(
+                  title: const Text('Sublist'),
+                  children: <Widget>[
+                    const ListTile(title: const Text('0')),
+                    const ListTile(title: const Text('1'))
+                  ],
+                  trailing: const Icon(Icons.inbox),
+                ),
+                const ListTile(title: const Text('Bottom'))
+              ]
+            )
+          )
+        );
+      }
+    };
+
+    await tester.pumpWidget(new MaterialApp(routes: routes));
+    expect(find.byIcon(Icons.inbox), findsOneWidget);
+    expect(find.byIcon(Icons.expand_more), findsNothing);
+  });
 }