Updated rectangle documentation to explain coordinate system.

sdk/lib/math/rectangle.dart

Index: sdk/lib/math/rectangle.dart
diff --git a/sdk/lib/math/rectangle.dart b/sdk/lib/math/rectangle.dart
index 0aa52ff79d1e61f707efc98a659124680c505765..05fb9385f469d8d7fee9a76fbae6ab03fa2dbb8f 100644
--- a/sdk/lib/math/rectangle.dart
+++ b/sdk/lib/math/rectangle.dart
@@ -5,6 +5,13 @@ part of dart.math;
 
 /**
  * A base class for representing two-dimensional axis-aligned rectangles.
+ *
+ * This rectangle uses a left-handed Cartesian coordinate system, with x
+ * directed to the right but y directed down, as per the convention in 2D

[floitsch, 2013/10/31 00:36:59]

s/but/and/

+ * computer graphics.
+ *
+ * See also:
+ *    [W3C Coordinate Systems Specification](http://www.w3.org/TR/SVG/coords.html#InitialCoordinateSystem).
  */
 abstract class _RectangleBase<T extends num> {
   const _RectangleBase();