Make tab's icon margin configurable (Fix #47363) (#47364)

Adds the `iconMargin` parameter to Tab

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

@@ -60,11 +60,14 @@ class Tab extends StatelessWidget {
   /// Creates a material design [TabBar] tab.
   ///
   /// At least one of [text], [icon], and [child] must be non-null. The [text]
-  /// and [child] arguments must not be used at the same time.
+  /// and [child] arguments must not be used at the same time. The
+  /// [iconMargin] is only useful when [icon] and either one of [text] or
+  /// [child] is non-null.
   const Tab({
     Key key,
     this.text,
     this.icon,
+    this.iconMargin = const EdgeInsets.only(bottom: 10.0),
     this.child,
   }) : assert(text != null || child != null || icon != null),
        assert(!(text != null && null != child)), // TODO(goderbauer): https://github.com/dart-lang/sdk/issues/34180
@@ -85,6 +88,12 @@ class Tab extends StatelessWidget {
   /// An icon to display as the tab's label.
   final Widget icon;
 
+  /// The margin added around the tab's icon.
+  ///
+  /// Only useful when used in combination with [icon], and either one of
+  /// [text] or [child] is non-null.
+  final EdgeInsetsGeometry iconMargin;
+
   Widget _buildLabelText() {
     return child ?? Text(text, softWrap: false, overflow: TextOverflow.fade);
   }
@@ -109,7 +118,7 @@ class Tab extends StatelessWidget {
         children: <Widget>[
           Container(
             child: icon,
-            margin: const EdgeInsets.only(bottom: 10.0),
+            margin: iconMargin,
           ),
           _buildLabelText(),
         ],

packages/flutter/test/material/tabs_test.dart

@@ -254,6 +254,30 @@ void main() {
     expect(tester.getSize(find.byType(Tab)), const Size(14.0, 72.0));
   }, skip: isBrowser);
 
+  testWidgets('Tab sizing - icon, iconMargin and text', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(fontFamily: 'Ahem'),
+        home: const Center(
+          child: Material(
+            child: Tab(
+              icon: SizedBox(
+                width: 10.0,
+                height: 10.0,
+              ),
+              iconMargin: EdgeInsets.symmetric(
+                horizontal: 100.0,
+              ),
+              text: 'x',
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(tester.renderObject<RenderParagraph>(find.byType(RichText)).text.style.fontFamily, 'Ahem');
+    expect(tester.getSize(find.byType(Tab)), const Size(210.0, 72.0));
+  }, skip: isBrowser);
+
   testWidgets('Tab sizing - icon and child', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(theme: ThemeData(fontFamily: 'Ahem'), home: const Center(child: Material(child: Tab(icon: SizedBox(width: 10.0, height: 10.0), child: Text('x'))))),