Add const to ClampingScrollPhysics. (#153)

Author: michdud

mdc_100_series/lib/supplemental/product_columns.dart

@@ -39,7 +39,7 @@ class TwoProductCardColumn extends StatelessWidget {
               : 33 / 49;
 
           return ListView(
-            physics: ClampingScrollPhysics(),
+            physics: const ClampingScrollPhysics(),
             children: <Widget>[
               Padding(
                 padding: EdgeInsetsDirectional.only(start: 28.0),
@@ -75,7 +75,7 @@ class OneProductCardColumn extends StatelessWidget {
   Widget build(BuildContext context) {
     return ListView(
       reverse: true,
-      physics: ClampingScrollPhysics(),
+      physics: const ClampingScrollPhysics(),
       children: <Widget>[
         SizedBox(
           height: 40.0,