Updated rectangle documentation to explain coordinate system.

sdk/lib/math/rectangle.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 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();
 
   /** The x-coordinate of the left edge. */
   T get left;
   /** The y-coordinate of the top edge. */
   T get top;
   /** The `width` of the rectangle. */
   T get width;
@@ skipping 130 matching lines @@
   MutableRectangle(this.left, this.top, this.width, this.height);
 
   factory MutableRectangle.fromPoints(Point<T> a, Point<T> b) {
     T left = min(a.x, b.x);
     T width = max(a.x, b.x) - left;
     T top = min(a.y, b.y);
     T height = max(a.y, b.y) - top;
     return new MutableRectangle<T>(left, top, width, height);
   }
 }