Use const constructor for RoundedRectangleBorder. (#14384)

This triggered an analysis failure only on the devicelab.

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

diff --git a/packages/flutter/lib/src/material/material.dart b/packages/flutter/lib/src/material/material.dart
index ac65cec..4ea8aca 100644
--- a/packages/flutter/lib/src/material/material.dart
+++ b/packages/flutter/lib/src/material/material.dart
@@ -332,7 +332,7 @@
     switch (widget.type) {
       case MaterialType.canvas:
       case MaterialType.transparency:
-        return new RoundedRectangleBorder();
+        return const RoundedRectangleBorder();
 
       case MaterialType.card:
       case MaterialType.button:
@@ -343,7 +343,7 @@
       case MaterialType.circle:
         return const CircleBorder();
     }
-    return new RoundedRectangleBorder();
+    return const RoundedRectangleBorder();
   }
 }