Custom offset functionality for Popup Menu Button (#22534)

* Adds offset functionality to popup menu
* Adds a test

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

@@ -820,7 +820,9 @@ class PopupMenuButton<T> extends StatefulWidget {
     this.padding = const EdgeInsets.all(8.0),
     this.child,
     this.icon,
+    this.offset = Offset.zero,
   }) : assert(itemBuilder != null),
+       assert(offset != null),
        assert(!(child != null && icon != null)), // fails if passed both parameters
        super(key: key);
 
@@ -864,6 +866,12 @@ class PopupMenuButton<T> extends StatefulWidget {
   /// If provided, the icon used for this button.
   final Icon icon;
 
+  /// The offset applied to the Popup Menu Button.
+  ///
+  /// When not set, the Popup Menu Button will be positioned directly next to
+  /// the button that was used to create it.
+  final Offset offset;
+
   @override
   _PopupMenuButtonState<T> createState() => _PopupMenuButtonState<T>();
 }
@@ -874,7 +882,7 @@ class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
     final RenderBox overlay = Overlay.of(context).context.findRenderObject();
     final RelativeRect position = RelativeRect.fromRect(
       Rect.fromPoints(
-        button.localToGlobal(Offset.zero, ancestor: overlay),
+        button.localToGlobal(widget.offset, ancestor: overlay),
         button.localToGlobal(button.size.bottomRight(Offset.zero), ancestor: overlay),
       ),
       Offset.zero & overlay.size,

packages/flutter/test/material/popup_menu_test.dart

@@ -430,6 +430,45 @@ void main() {
     expect(MediaQuery.of(popupContext).padding, EdgeInsets.zero);
   });
 
+  testWidgets('Popup Menu Offset Test', (WidgetTester tester) async {
+    const Offset offset = Offset(100.0, 100.0);
+
+    final PopupMenuButton<int> popupMenuButton =
+      PopupMenuButton<int>(
+        offset: offset,
+        itemBuilder: (BuildContext context) {
+          return <PopupMenuItem<int>>[
+            PopupMenuItem<int>(
+              value: 1,
+              child: Builder(
+                builder: (BuildContext context) {
+                  return const Text('AAA');
+                },
+              ),
+            ),
+          ];
+        },
+      );
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: Material(
+              child: popupMenuButton,
+            ),
+          )
+        ),
+      ),
+    );
+
+    await tester.tap(find.byType(IconButton));
+    await tester.pumpAndSettle();
+
+    // The position is different than the offset because the default position isn't at the origin.
+    expect(tester.getTopLeft(find.byWidgetPredicate((Widget w) => '${w.runtimeType}' == '_PopupMenu<int>')), const Offset(364.0, 324.0));
+  });
+
   testWidgets('open PopupMenu has correct semantics', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);
     await tester.pumpWidget(MaterialApp(