PopupMenuButton: create IconButton if child is Icon (#10230)

* PopupMenuButton: create IconButton if child is Icon

Otherwise the resulting button has an abnormally small and rectangular
area. With multiple PopupMenuButton(child: Icon) they get squished
together in the AppBar.

* Add separate icon argument to PopupMenuButton

* Fix style issues and tweak dartdocs

* Add tests for icon argument to PopupMenuButton

* Group icon tests and fix broken test, analyzer warnings

* Test that the correct custom icon is present

* Apply De Morgan's to work around dart analyzer bug

see: https://github.com/dart-lang/sdk/issues/30288

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

@@ -617,8 +617,13 @@ typedef List<PopupMenuEntry<T>> PopupMenuItemBuilder<T>(BuildContext context);
 
 /// Displays a menu when pressed and calls [onSelected] when the menu is dismissed
 /// because an item was selected. The value passed to [onSelected] is the value of
-/// the selected menu item. If child is null then a standard 'navigation/more_vert'
-/// icon is created.
+/// the selected menu item.
+///
+/// One of [child] or [icon] may be provided, but not both. If [icon] is provided,
+/// then [PopupMenuButton] behaves like an [IconButton].
+///
+/// If both are null, then a standard overflow icon is created (depending on the
+/// platform).
 ///
 /// ## Sample code
 ///
@@ -672,8 +677,10 @@ class PopupMenuButton<T> extends StatefulWidget {
     this.tooltip: 'Show menu',
     this.elevation: 8.0,
     this.padding: const EdgeInsets.all(8.0),
-    this.child
+    this.child,
+    this.icon,
   }) : assert(itemBuilder != null),
+       assert(!(child != null && icon != null)), // fails if passed both parameters
        super(key: key);
 
   /// Called when the button is pressed to create the items to show in the menu.
@@ -702,9 +709,12 @@ class PopupMenuButton<T> extends StatefulWidget {
   /// to set the padding to zero.
   final EdgeInsets padding;
 
-  /// The widget below this widget in the tree.
+  /// If provided, the widget used for this button.
   final Widget child;
 
+  /// If provided, the icon used for this button.
+  final Icon icon;
+
   @override
   _PopupMenuButtonState<T> createState() => new _PopupMenuButtonState<T>();
 }
@@ -747,17 +757,16 @@ class _PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
 
   @override
   Widget build(BuildContext context) {
-    if (widget.child == null) {
-      return new IconButton(
-        icon: _getIcon(Theme.of(context).platform),
+    return widget.child != null
+      ? new InkWell(
+          onTap: showButtonMenu,
+          child: widget.child,
+        )
+      : new IconButton(
+          icon: widget.icon ?? _getIcon(Theme.of(context).platform),
           padding: widget.padding,
           tooltip: widget.tooltip,
           onPressed: showButtonMenu,
         );
   }
-    return new InkWell(
-      onTap: showButtonMenu,
-      child: widget.child,
-    );
-  }
 }

packages/flutter/test/material/popup_menu_test.dart

@@ -90,4 +90,46 @@ void main() {
     expect(find.byIcon(Icons.more_vert), findsNothing);
     expect(find.byIcon(Icons.more_horiz), findsOneWidget);
   });
+
+  group('PopupMenuButton with Icon', () {
+
+    // Helper function to create simple and valid popup menus.
+    List<PopupMenuItem<int>> simplePopupMenuItemBuilder(BuildContext context) {
+      return <PopupMenuItem<int>>[
+        const PopupMenuItem<int>(
+            value: 1,
+            child: const Text('1'),
+        ),
+      ];
+    }
+
+    testWidgets('PopupMenuButton fails when given both child and icon', (WidgetTester tester) async {
+      expect(() {
+        new PopupMenuButton<int>(
+            child: const Text('heyo'),
+            icon: const Icon(Icons.view_carousel),
+            itemBuilder: simplePopupMenuItemBuilder,
+        );
+      }, throwsA(new isInstanceOf<AssertionError>()));
+    });
+
+    testWidgets('PopupMenuButton creates IconButton when given an icon', (WidgetTester tester) async {
+      final PopupMenuButton<int> button = new PopupMenuButton<int>(
+        icon: const Icon(Icons.view_carousel),
+        itemBuilder: simplePopupMenuItemBuilder,
+      );
+
+      await tester.pumpWidget(new MaterialApp(
+          home: new Scaffold(
+            appBar: new AppBar(
+              actions: <Widget>[button],
+            ),
+          ),
+        ),
+      );
+
+      expect(find.byType(IconButton), findsOneWidget);
+      expect(find.byIcon(Icons.view_carousel), findsOneWidget);
+    });
+  });
 }