Add the floating action button to stocks2

sky/engine/core/painting/Size.dart

Index: sky/engine/core/painting/Size.dart
diff --git a/sky/engine/core/painting/Size.dart b/sky/engine/core/painting/Size.dart
index 6747a65523e8d4ebf154b6a5ecf6e36f769f417f..139fc16db171bcf660afca5f77f5fb3530197b46 100644
--- a/sky/engine/core/painting/Size.dart
+++ b/sky/engine/core/painting/Size.dart
@@ -18,6 +18,8 @@ class Size {
   static const Size infinite = const Size(double.INFINITY, double.INFINITY);
 
   bool operator ==(other) => other is Size && width == other.width && height == other.height;
+  Size operator +(Size other) => new Size(width + other.width, height + other.height);
+  Size operator -(Size other) => new Size(width - other.width, height - other.height);
 
   // does the equivalent of "return new Point(0,0) + this"
   Point toPoint() => new Point(this.width, this.height);