Add support for font variation based theming to `Icon` (#109140)

packages/flutter/lib/src/widgets/icon.dart

@@ -240,6 +240,14 @@ class Icon extends StatelessWidget {
 
     final double? iconSize = size ?? iconTheme.size;
 
+    final double? iconFill = fill ?? iconTheme.fill;
+
+    final double? iconWeight = weight ?? iconTheme.weight;
+
+    final double? iconGrade = grade ?? iconTheme.grade;
+
+    final double? iconOpticalSize = opticalSize ?? iconTheme.opticalSize;
+
     final List<Shadow>? iconShadows = shadows ?? iconTheme.shadows;
 
     if (icon == null) {
@@ -262,10 +270,10 @@ class Icon extends StatelessWidget {
         text: String.fromCharCode(icon!.codePoint),
         style: TextStyle(
           fontVariations: <FontVariation>[
-            if (fill != null) FontVariation('FILL', fill!),
-            if (weight != null) FontVariation('wght', weight!),
-            if (grade != null) FontVariation('GRAD', grade!),
-            if (opticalSize != null) FontVariation('opsz', opticalSize!),
+            if (iconFill != null) FontVariation('FILL', iconFill),
+            if (iconWeight != null) FontVariation('wght', iconWeight),
+            if (iconGrade != null) FontVariation('GRAD', iconGrade),
+            if (iconOpticalSize != null) FontVariation('opsz', iconOpticalSize),
           ],
           inherit: false,
           color: iconColor,

packages/flutter/test/widgets/icon_test.dart

@@ -220,7 +220,12 @@ void main() {
     );
 
     RichText text = tester.widget(find.byType(RichText));
-    expect(text.text.style!.fontVariations, <FontVariation>[]);
+    expect(text.text.style!.fontVariations, <FontVariation>[
+      const FontVariation('FILL', 0.0),
+      const FontVariation('wght', 400.0),
+      const FontVariation('GRAD', 0.0),
+      const FontVariation('opsz', 48.0)
+    ]);
 
     await tester.pumpWidget(
       const Directionality(
@@ -239,6 +244,57 @@ void main() {
     ]);
   });
 
+  testWidgets('Fill, weight, grade, and optical size can be set at the theme-level', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: IconTheme(
+          data: IconThemeData(
+            fill: 0.2,
+            weight: 3.0,
+            grade: 4.0,
+            opticalSize: 5.0,
+          ),
+          child: Icon(Icons.abc),
+        ),
+      ),
+    );
+
+    final RichText text = tester.widget(find.byType(RichText));
+    expect(text.text.style!.fontVariations, <FontVariation>[
+      const FontVariation('FILL', 0.2),
+      const FontVariation('wght', 3.0),
+      const FontVariation('GRAD', 4.0),
+      const FontVariation('opsz', 5.0)
+    ]);
+  });
+
+  testWidgets('Theme-level fill, weight, grade, and optical size can be overriden', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: IconTheme(
+          data: IconThemeData(
+            fill: 0.2,
+            weight: 3.0,
+            grade: 4.0,
+            opticalSize: 5.0,
+          ),
+          child: Icon(Icons.abc, fill: 0.6, weight: 7.0, grade: 8.0, opticalSize: 9.0),
+        ),
+      ),
+    );
+
+    final RichText text = tester.widget(find.byType(RichText));
+    expect(text.text.style!.fontVariations, isNotNull);
+    expect(text.text.style!.fontVariations, <FontVariation>[
+      const FontVariation('FILL', 0.6),
+      const FontVariation('wght', 7.0),
+      const FontVariation('GRAD', 8.0),
+      const FontVariation('opsz', 9.0)
+    ]);
+  });
+
   test('Throws if given invalid values', () {
     expect(() => Icon(Icons.abc, fill: -0.1), throwsAssertionError);
     expect(() => Icon(Icons.abc, fill: 1.1), throwsAssertionError);