Made the showMenu() position parameter required (#30206)

Made the showMenu() position parameter required as it doesn't make sense to show a menu without indicating where it should be shown. Also added a test to verify this.

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

@@ -712,13 +712,14 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
 ///    semantics.
 Future<T> showMenu<T>({
   @required BuildContext context,
-  RelativeRect position,
+  @required RelativeRect position,
   @required List<PopupMenuEntry<T>> items,
   T initialValue,
   double elevation = 8.0,
   String semanticLabel,
 }) {
   assert(context != null);
+  assert(position != null);
   assert(items != null && items.isNotEmpty);
   assert(debugCheckHasMaterialLocalizations(context));
   String label = semanticLabel;

packages/flutter/test/material/popup_menu_test.dart

@@ -606,6 +606,41 @@ void main() {
     expect(selectedValue, '2');
   });
 
+  testWidgets('showMenu position required', (WidgetTester tester) async {
+    // Test for https://github.com/flutter/flutter/issues/22256
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: Center(
+            child: Builder(
+              builder: (BuildContext context) {
+                return RaisedButton(
+                  onPressed: () {
+                    // Ensure showMenu throws an assertion without a position
+                    expect(() {
+                      // ignore: missing_required_param
+                      showMenu<int>(
+                        context: context,
+                        items: <PopupMenuItem<int>>[
+                          const PopupMenuItem<int>(
+                              value: 1, child: Text('1')
+                          ),
+                        ],
+                      );
+                    }, throwsAssertionError);
+                  },
+                  child: const Text('Menu Button'),
+                );
+              },
+            ),
+          ),
+        ),
+      )
+    );
+
+    await tester.tap(find.text('Menu Button'));
+  });
+
 }
 
 class TestApp extends StatefulWidget {