increase checkbox size and fix header area (#19966)

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

@@ -364,13 +364,16 @@ class DataTable extends StatelessWidget {
     VoidCallback onRowTap,
     ValueChanged<bool> onCheckboxChanged
   }) {
-    Widget contents = new Padding(
-      padding: const EdgeInsetsDirectional.only(start: _tablePadding, end: _tablePadding / 2.0),
-      child: new Center(
-        child: new Checkbox(
-          activeColor: color,
-          value: checked,
-          onChanged: onCheckboxChanged,
+    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(
+            activeColor: color,
+            value: checked,
+            onChanged: onCheckboxChanged,
+          ),
         ),
       ),
     );

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

@@ -380,34 +380,38 @@ class PaginatedDataTableState extends State<PaginatedDataTable> {
 
     // CARD
     return new Card(
+      semanticContainer: false,
       child: new Column(
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: <Widget>[
-          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
-            style: _selectedRowCount > 0 ? themeData.textTheme.subhead.copyWith(color: themeData.accentColor)
-                                         : themeData.textTheme.title.copyWith(fontWeight: FontWeight.w400),
-            child: IconTheme.merge(
-              data: const IconThemeData(
-                opacity: 0.54
-              ),
-              child: new ButtonTheme.bar(
-                child: new Container(
-                  height: 64.0,
-                  padding: new EdgeInsetsDirectional.only(start: startPadding, end: 14.0),
-                  // TODO(ianh): This decoration will prevent ink splashes from being visible.
-                  // Instead, we should have a widget that prints the decoration on the material.
-                  // See https://github.com/flutter/flutter/issues/3782
-                  color: _selectedRowCount > 0 ? themeData.secondaryHeaderColor : null,
-                  child: new Row(
-                    mainAxisAlignment: MainAxisAlignment.end,
-                    children: headerWidgets
+          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
+              style: _selectedRowCount > 0 ? themeData.textTheme.subhead.copyWith(color: themeData.accentColor)
+                                           : themeData.textTheme.title.copyWith(fontWeight: FontWeight.w400),
+              child: IconTheme.merge(
+                data: const IconThemeData(
+                  opacity: 0.54
+                ),
+                child: new ButtonTheme.bar(
+                  child: new Container(
+                    height: 64.0,
+                    padding: new EdgeInsetsDirectional.only(start: startPadding, end: 14.0),
+                    // TODO(ianh): This decoration will prevent ink splashes from being visible.
+                    // Instead, we should have a widget that prints the decoration on the material.
+                    // See https://github.com/flutter/flutter/issues/3782
+                    color: _selectedRowCount > 0 ? themeData.secondaryHeaderColor : null,
+                    child: new Row(
+                      mainAxisAlignment: MainAxisAlignment.end,
+                      children: headerWidgets
+                    )
                   )
                 )
               )
-            )
+            ),
           ),
           new SingleChildScrollView(
             scrollDirection: Axis.horizontal,