increase checkbox size and fix header area (#19966)

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

@@ -364,7 +364,9 @@ class DataTable extends StatelessWidget {
     VoidCallback onRowTap,
     ValueChanged<bool> onCheckboxChanged
   }) {
-    Widget contents = new Padding(
+    Widget contents = new Semantics(
+      container: true,
+      child: new Padding(
         padding: const EdgeInsetsDirectional.only(start: _tablePadding, end: _tablePadding / 2.0),
         child: new Center(
           child: new Checkbox(
@@ -373,6 +375,7 @@ class DataTable extends StatelessWidget {
             onChanged: onCheckboxChanged,
           ),
         ),
+      ),
     );
     if (onRowTap != null) {
       contents = new TableRowInkWell(

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

@@ -380,10 +380,13 @@ class PaginatedDataTableState extends State<PaginatedDataTable> {
 
     // CARD
     return new Card(
+      semanticContainer: false,
       child: new Column(
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: <Widget>[
-          new DefaultTextStyle(
+          new Semantics(
+            container: true,
+            child: new DefaultTextStyle(
               // These typographic styles aren't quite the regular ones. We pick the closest ones from the regular
               // list and then tweak them appropriately.
               // See https://material.google.com/components/data-tables.html#data-tables-tables-within-cards
@@ -409,6 +412,7 @@ class PaginatedDataTableState extends State<PaginatedDataTable> {
                 )
               )
             ),
+          ),
           new SingleChildScrollView(
             scrollDirection: Axis.horizontal,
             child: new DataTable(