Support hairline borders

Previously, border with '0' was ambiguous. Sometimes we treated it as
hairline borders, sometimes as "don't show the border", though even in
the latter case we did some graphics work sometimes. Now we have an
explicit BorderStyle.none flag to not draw the border efficiently.

dev/manual_tests/drag_and_drop.dart

@@ -64,7 +64,7 @@ class DotState extends State<Dot> {
         height: config.size,
         decoration: new BoxDecoration(
           backgroundColor: config.color,
-          border: new Border.all(color: const Color(0xFF000000), width: taps.toDouble()),
+          border: new Border.all(width: taps.toDouble()),
           shape: BoxShape.circle
         ),
         child: config.child