Cleanup OutlinedButton.icon documentation and implementation (#176630)

## Description

This PR cleanup `OutlinedButton.icon` documentation and recent logic
change from https://github.com/flutter/flutter/pull/175810.

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

@@ -86,13 +86,14 @@ class OutlinedButton extends ButtonStyleButton {
     required super.child,
   }) : _addPadding = false;
 
-  /// Create a text button from a pair of widgets that serve as the button's
+  /// Create an outlined button from a pair of widgets that serve as the button's
   /// [icon] and [label].
   ///
   /// The icon and label are arranged in a row and padded by 12 logical pixels
   /// at the start, and 16 at the end, with an 8 pixel gap in between.
   ///
-  /// If [icon] is null, will create an [OutlinedButton] instead.
+  /// If [icon] is null, this constructor will create an outlined button
+  /// that doesn't display an icon.
   ///
   /// {@macro flutter.material.ButtonStyleButton.iconAlignment}
   ///
@@ -114,10 +115,10 @@ class OutlinedButton extends ButtonStyleButton {
        super(
          child: icon != null
              ? _OutlinedButtonWithIconChild(
-                 iconAlignment: iconAlignment,
                  label: label,
-                 buttonStyle: style,
                  icon: icon,
+                 buttonStyle: style,
+                 iconAlignment: iconAlignment,
                )
              : label,
        );
@@ -419,13 +420,13 @@ EdgeInsetsGeometry _scaledPadding(BuildContext context) {
 class _OutlinedButtonWithIconChild extends StatelessWidget {
   const _OutlinedButtonWithIconChild({
     required this.label,
-    this.icon,
+    required this.icon,
     required this.buttonStyle,
     required this.iconAlignment,
   });
 
   final Widget label;
-  final Widget? icon;
+  final Widget icon;
   final ButtonStyle? buttonStyle;
   final IconAlignment? iconAlignment;
 
@@ -442,12 +443,6 @@ class _OutlinedButtonWithIconChild extends StatelessWidget {
         outlinedButtonTheme.style?.iconAlignment ??
         buttonStyle?.iconAlignment ??
         IconAlignment.start;
-    final Widget? icon = this.icon;
-
-    if (icon == null) {
-      return label;
-    }
-
     return Row(
       mainAxisSize: MainAxisSize.min,
       spacing: lerpDouble(8, 4, scale)!,

packages/flutter/test/material/outlined_button_test.dart

@@ -1295,7 +1295,7 @@ void main() {
     // Initially, no icons are present.
     expect(find.byIcon(Icons.favorite), findsNothing);
 
-    // Find the original OutlinedButton with no icon and get it's SemanticsNode
+    // Find the original OutlinedButton with no icon and get its SemanticsNode.
     final Finder outlinedButton = find.bySemanticsLabel('Button');
     expect(outlinedButton, findsOneWidget);
 
@@ -1308,7 +1308,7 @@ void main() {
     // Now one icon should be present.
     expect(find.byIcon(Icons.favorite), findsOneWidget);
 
-    // Check if the semantics has change
+    // Check if the semantics has change.
     final SemanticsNode semanticsNodeWithIcon = tester.getSemantics(outlinedButton);
 
     expect(semanticsNodeWithIcon, origSemanticsNode);
@@ -3047,4 +3047,33 @@ void main() {
       matchesGoldenFile('outlined_button.badge.outline.png'),
     );
   });
+
+  testWidgets('OutlinedButton.icon does not lose focus when icon is nullified', (
+    WidgetTester tester,
+  ) async {
+    Widget buildButton({required Widget? icon}) {
+      return MaterialApp(
+        home: Center(
+          child: OutlinedButton.icon(onPressed: () {}, icon: icon, label: const Text('button')),
+        ),
+      );
+    }
+
+    // Build once with an icon.
+    await tester.pumpWidget(buildButton(icon: const Icon(Icons.abc)));
+
+    FocusNode getButtonFocusNode() {
+      return Focus.of(tester.element(find.text('button')));
+    }
+
+    getButtonFocusNode().requestFocus();
+    await tester.pumpAndSettle();
+    expect(getButtonFocusNode().hasFocus, true);
+
+    // Rebuild without icon.
+    await tester.pumpWidget(buildButton(icon: null));
+
+    // The button should still be focused.
+    expect(getButtonFocusNode().hasFocus, true);
+  });
 }