Keep Icon's subtree structure the same with and without a semanticLabel. (#12516)

* Keep Icon's subtree structure the same with and without a semanticLabel.

This avoids rebuilding the subtree when a semanticLabel is set/unset.
Also updated some doc comments.

(This is a followup on post-merge comments for #12475)

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

@@ -89,7 +89,10 @@ class Icon extends StatelessWidget {
   /// This would be read out in accessibility modes (e.g TalkBack/VoiceOver).
   /// This label does not show in the UI.
   ///
-  /// See [Semantics.label];
+  /// See also:
+  ///
+  ///  * [Semantics.label] which is set with [semanticLabel] in the underlying
+  ///    [Semantics] widget.
   final String semanticLabel;
 
   @override
@@ -102,15 +105,19 @@ class Icon extends StatelessWidget {
     final double iconSize = size ?? iconTheme.size;
 
     if (icon == null)
-      return _wrapWithSemantics(new SizedBox(width: iconSize, height: iconSize));
+      return new Semantics(
+        label: semanticLabel,
+        child: new SizedBox(width: iconSize, height: iconSize)
+      );
 
     final double iconOpacity = iconTheme.opacity;
     Color iconColor = color ?? iconTheme.color;
     if (iconOpacity != 1.0)
       iconColor = iconColor.withOpacity(iconColor.opacity * iconOpacity);
 
-    return _wrapWithSemantics(
-      new ExcludeSemantics(
+    return new Semantics(
+      label: semanticLabel,
+      child: new ExcludeSemantics(
         child: new SizedBox(
           width: iconSize,
           height: iconSize,
@@ -133,17 +140,6 @@ class Icon extends StatelessWidget {
     );
   }
 
-  /// Wraps the widget with a Semantics widget if [semanticLabel] is set.
-  Widget _wrapWithSemantics(Widget widget) {
-    if (semanticLabel == null)
-      return widget;
-
-    return new Semantics(
-      child: widget,
-      label: semanticLabel,
-    );
-  }
-
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder description) {
     super.debugFillProperties(description);

packages/flutter/test/widgets/icon_test.dart

@@ -161,4 +161,35 @@ void main() {
 
     expect(semantics, hasSemantics(new TestSemantics.root(label: 'a label')));
   });
+
+  testWidgets('Changing semantic label from null doesn\'t rebuild tree ', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: const Center(
+          child: const Icon(Icons.time_to_leave),
+        ),
+      ),
+    );
+
+    final Element richText1 = tester.element(find.byType(RichText));
+
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: const Center(
+          child: const Icon(
+            Icons.time_to_leave,
+            semanticLabel: 'a label',
+          ),
+        ),
+      ),
+    );
+
+    final Element richText2 = tester.element(find.byType(RichText));
+
+    // Compare a leaf Element in the Icon subtree before and after changing the
+    // semanticLabel to make sure the subtree was not rebuilt.
+    expect(richText2, same(richText1));
+  });
 }