Provide an initial rotation of the ExpansionIcon if isExpanded = true on initState (#15657)

* rotate expansion icon fully if initial state expanded = true

* add test case for initially expanded

* remove unused import

* Update expand_icon_test.dart

add linebreak

* import dart:math as math

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

 // Copyright 2015 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 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
@@ -72,6 +73,10 @@ class _ExpandIconState extends State<ExpandIcon> with SingleTickerProviderStateM
         curve: Curves.fastOutSlowIn
       )
     );
+    // If the widget is initially expanded, rotate the icon without animating it.
+    if (widget.isExpanded) {
+      _controller.value = math.pi;
+    }
   }
 
   @override

packages/flutter/test/material/expand_icon_test.dart

@@ -67,6 +67,23 @@ void main() {
 
     expect(expanded, isFalse);
   });
+
+  testWidgets('ExpandIcon is rotated initially if isExpanded is true on first build', (WidgetTester tester) async {
+    bool expanded = true;
+
+    await tester.pumpWidget(
+        wrap(
+            child: new ExpandIcon(
+              isExpanded: expanded,
+              onPressed: (bool isExpanded) {
+                expanded = !isExpanded;
+              },
+            )
+        )
+    );
+    final RotationTransition rotation = tester.firstWidget(find.byType(RotationTransition));
+    expect(rotation.turns.value, 0.5);
+  });
 }
 
 Widget wrap({ Widget child }) {