Add the ability to draw borders on circles

We now support uniform borders on circular box decorations.

Fixes #741

examples/widgets/big_circle.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:sky/widgets.dart';
+import 'package:sky/theme/colors.dart' as colors;
+
+class BigCircleApp extends App {
+  Widget build() {
+    return new Container(
+        padding: new EdgeDims.all(50.0),
+        decoration: new BoxDecoration(
+          shape: Shape.circle,
+          border: new Border.all(width: 10.0, color: const Color(0x80FF00FF)),
+          backgroundColor: colors.Teal[600]
+        )
+    );
+  }
+}
+
+void main() {
+  runApp(new BigCircleApp());
+}

packages/flutter/lib/painting/box_painter.dart

@@ -516,10 +516,16 @@ class BoxPainter {
     if (_decoration.border == null)
       return;
 
-    if (_hasUniformBorder && _decoration.borderRadius != null) {
+    if (_hasUniformBorder) {
+      if (_decoration.borderRadius != null) {
         _paintBorderWithRadius(canvas, rect);
         return;
       }
+      if (_decoration.shape == Shape.circle) {
+        _paintBorderWithCircle(canvas, rect);
+        return;
+      }
+    }
 
     assert(_decoration.borderRadius == null); // TODO(abarth): Support non-uniform rounded borders.
     assert(_decoration.shape == Shape.rectangle); // TODO(ianh): Support borders on circles.
@@ -571,6 +577,7 @@ class BoxPainter {
 
   void _paintBorderWithRadius(sky.Canvas canvas, Rect rect) {
     assert(_hasUniformBorder);
+    assert(_decoration.shape == Shape.rectangle);
     Color color = _decoration.border.top.color;
     double width = _decoration.border.top.width;
     double radius = _decoration.borderRadius;
@@ -580,6 +587,20 @@ class BoxPainter {
     canvas.drawDRRect(outer, inner, new Paint()..color = color);
   }
 
+  void _paintBorderWithCircle(sky.Canvas canvas, Rect rect) {
+    assert(_hasUniformBorder);
+    assert(_decoration.shape == Shape.circle);
+    assert(_decoration.borderRadius == null);
+    double width = _decoration.border.top.width;
+    Paint paint = new Paint()
+      ..color = _decoration.border.top.color
+      ..strokeWidth = width
+      ..setStyle(sky.PaintingStyle.stroke);
+    Point center = rect.center;
+    double radius = (rect.shortestSide - width) / 2.0;
+    canvas.drawCircle(center, radius, paint);
+  }
+
   void paint(sky.Canvas canvas, Rect rect) {
     _paintBackgroundColor(canvas, rect);
     _paintBackgroundImage(canvas, rect);