Add operator+ and operator- to EdgeDims

... and use them to improve the horizontal_scrolling example.

packages/flutter/example/widgets/horizontal_scrolling.dart

@@ -6,10 +6,14 @@ import 'package:sky/painting/box_painter.dart';
 import 'package:sky/widgets.dart';
 
 class Circle extends Component {
+  Circle({ this.margin: EdgeDims.zero });
+
+  final EdgeDims margin;
+
   Widget build() {
     return new Container(
       width: 50.0,
-      margin: new EdgeDims.symmetric(horizontal: 2.0),
+      margin: margin + new EdgeDims.symmetric(horizontal: 2.0),
       decoration: new BoxDecoration(
         shape: Shape.circle,
         backgroundColor: const Color(0xFF00FF00)
@@ -18,18 +22,10 @@ class Circle extends Component {
   }
 }
 
-class Pad extends Component {
-  Widget build() {
-    return new SizedBox(width: 10.0);
-  }
-}
-
 class HorizontalScrollingApp extends App {
   Widget build() {
     List<Widget> circles = [
-      new Pad(),
-      new Circle(),
-      new Circle(),
+      new Circle(margin: new EdgeDims.only(left: 10.0)),
       new Circle(),
       new Circle(),
       new Circle(),
@@ -40,7 +36,7 @@ class HorizontalScrollingApp extends App {
       new Circle(),
       new Circle(),
       new Circle(),
-      new Pad(),
+      new Circle(margin: new EdgeDims.only(right: 10.0)),
     ];
 
     return new Center(

packages/flutter/lib/rendering/box.dart

@@ -52,6 +52,22 @@ class EdgeDims {
         && left == other.left;
   }
 
+  EdgeDims operator+(EdgeDims other) {
+    return new EdgeDims(top + other.top,
+                        right + other.right,
+                        bottom + other.bottom,
+                        left + other.left);
+  }
+
+  EdgeDims operator-(EdgeDims other) {
+    return new EdgeDims(top - other.top,
+                        right - other.right,
+                        bottom - other.bottom,
+                        left - other.left);
+  }
+
+  static const EdgeDims zero = const EdgeDims(0.0, 0.0, 0.0, 0.0);
+
   int get hashCode {
     int value = 373;
     value = 37 * value + top.hashCode;