move semantics inside of opacity animation (#19960)

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

@@ -458,13 +458,19 @@ class _PopupMenu<T> extends StatelessWidget {
       ),
       child: new IntrinsicWidth(
         stepWidth: _kMenuWidthStep,
+        child: new Semantics(
+          scopesRoute: true,
+          namesRoute: true,
+          explicitChildNodes: true,
+          label: semanticLabel,
           child: new SingleChildScrollView(
             padding: const EdgeInsets.symmetric(
               vertical: _kMenuVerticalPadding
             ),
             child: new ListBody(children: children),
-        )
-      )
+          ),
+        ),
+      ),
     );
 
     return new AnimatedBuilder(
@@ -479,15 +485,9 @@ class _PopupMenu<T> extends StatelessWidget {
               alignment: AlignmentDirectional.topEnd,
               widthFactor: width.evaluate(route.animation),
               heightFactor: height.evaluate(route.animation),
-              child: new Semantics(
-                scopesRoute: true,
-                namesRoute: true,
-                explicitChildNodes: true,
-                label: semanticLabel,
               child: child,
             ),
           ),
-          ),
         );
       },
       child: child,