_SaltedKey solution to `ExpansionPanelList` (#11902)

* _SaltedKey solution to `ExpansionPanelList`

_SaltedKey implementation courtesy of @Hixie
Tested and confirmed working.
Fixes #11166

* Added a simple test

* Style correction to test

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

@@ -13,6 +13,32 @@ import 'theme.dart';
 const double _kPanelHeaderCollapsedHeight = 48.0;
 const double _kPanelHeaderExpandedHeight = 64.0;
 
+class _SaltedKey<S, V> extends LocalKey {
+  const _SaltedKey(this.salt, this.value);
+
+  final S salt;
+  final V value;
+
+  @override
+  bool operator ==(dynamic other) {
+    if (other.runtimeType != runtimeType)
+      return false;
+    final _SaltedKey<S, V> typedOther = other;
+    return salt == typedOther.salt
+        && value == typedOther.value;
+  }
+
+  @override
+  int get hashCode => hashValues(runtimeType, salt, value);
+
+  @override
+  String toString() {
+    final String saltString = S == String ? '<\'$salt\'>' : '<$salt>';
+    final String valueString = V == String ? '<\'$value\'>' : '<$value>';
+    return '[$saltString $valueString]';
+  }
+}
+
 /// Signature for the callback that's called when an [ExpansionPanel] is
 /// expanded or collapsed.
 ///
@@ -109,7 +135,7 @@ class ExpansionPanelList extends StatelessWidget {
 
     for (int i = 0; i < children.length; i += 1) {
       if (_isChildExpanded(i) && i != 0 && !_isChildExpanded(i - 1))
-        items.add(new MaterialGap(key: new ValueKey<int>(i * 2 - 1)));
+        items.add(new MaterialGap(key: new _SaltedKey<BuildContext, int>(context, i * 2 - 1)));
 
       final Row header = new Row(
         children: <Widget>[
@@ -144,7 +170,7 @@ class ExpansionPanelList extends StatelessWidget {
 
       items.add(
         new MaterialSlice(
-          key: new ValueKey<int>(i * 2),
+          key: new _SaltedKey<BuildContext, int>(context, i * 2),
           child: new Column(
             children: <Widget>[
               header,
@@ -163,7 +189,7 @@ class ExpansionPanelList extends StatelessWidget {
       );
 
       if (_isChildExpanded(i) && i != children.length - 1)
-        items.add(new MaterialGap(key: new ValueKey<int>(i * 2 + 1)));
+        items.add(new MaterialGap(key: new _SaltedKey<BuildContext, int>(context, i * 2 + 1)));
     }
 
     return new MergeableMaterial(

packages/flutter/test/material/expansion_panel_test.dart

@@ -71,4 +71,42 @@ void main() {
     box = tester.renderObject(find.byType(ExpansionPanelList));
     expect(box.size.height - oldHeight, greaterThanOrEqualTo(100.0)); // 100 + some margin
   });
+  testWidgets("Multiple Panel List test", (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new ListView(
+          children: <ExpansionPanelList>[
+            new ExpansionPanelList(
+              children: <ExpansionPanel>[
+                new ExpansionPanel(
+                  headerBuilder: (BuildContext context, bool isExpanded) {
+                    return new Text(isExpanded ? 'B' : 'A');
+                  },
+                  body: const SizedBox(height:100.0),
+                  isExpanded: true,
+                ),
+              ],
+            ),
+            new ExpansionPanelList(
+              children: <ExpansionPanel>[
+                new ExpansionPanel(
+                  headerBuilder: (BuildContext context, bool isExpanded){
+                    return new Text(isExpanded ? 'D' : 'C');
+                  },
+                  body: const SizedBox(height:100.0),
+                  isExpanded: true,
+                ),
+              ],
+            ),
+          ],
+        ),
+      ),
+    );
+    await tester.pump(const Duration(milliseconds: 200));
+
+    expect(find.text('A'), findsNothing);
+    expect(find.text('B'), findsOneWidget);
+    expect(find.text('C'), findsNothing);
+    expect(find.text('D'), findsOneWidget);
+  });
 }