Add header SemanticsFlag to bottom nav bar (#19803)

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

@@ -277,6 +277,7 @@ class _BottomNavigationTile extends StatelessWidget {
       flex: size,
       child: new Semantics(
         container: true,
+        header: true,
         selected: selected,
         child: new Stack(
           children: <Widget>[

packages/flutter/test/material/bottom_navigation_bar_test.dart

@@ -574,19 +574,28 @@ void main() {
               children: <TestSemantics>[
                 new TestSemantics(
                   id: 3,
-                  flags: <SemanticsFlag>[SemanticsFlag.isSelected],
+                  flags: <SemanticsFlag>[
+                    SemanticsFlag.isSelected,
+                    SemanticsFlag.isHeader,
+                  ],
                   actions: <SemanticsAction>[SemanticsAction.tap],
                   label: 'AC\nTab 1 of 3',
                   textDirection: TextDirection.ltr,
                 ),
                 new TestSemantics(
                   id: 4,
+                  flags: <SemanticsFlag>[
+                    SemanticsFlag.isHeader,
+                  ],
                   actions: <SemanticsAction>[SemanticsAction.tap],
                   label: 'Alarm\nTab 2 of 3',
                   textDirection: TextDirection.ltr,
                 ),
                 new TestSemantics(
                   id: 5,
+                  flags: <SemanticsFlag>[
+                    SemanticsFlag.isHeader,
+                  ],
                   actions: <SemanticsAction>[SemanticsAction.tap],
                   label: 'Hot Tub\nTab 3 of 3',
                   textDirection: TextDirection.ltr,