Make Rectangle and MutableRectangle constructors handle negative lengths.

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 e0a3b9870fce43a6644f1baa255ffe8f6ca82ac0..fd6b7bec2b0e1a5ee9cb7f222a5bd33019937ee2 100644
--- a/sdk/lib/math/rectangle.dart
+++ b/sdk/lib/math/rectangle.dart
@@ -12,6 +12,10 @@ part of dart.math;
  *
  * See also:
  *    [W3C Coordinate Systems Specification](http://www.w3.org/TR/SVG/coords.html#InitialCoordinateSystem).
+ *
+ * The rectangle is the set of points with representable coordinates greater
+ * than or equal to left/top, and with distance to left/top no greater than
+ * width/height (to the limit of the precission of the coordinates).
  */
 abstract class _RectangleBase<T extends num> {
   const _RectangleBase();
@@ -130,7 +134,11 @@ class Rectangle<T extends num> extends _RectangleBase<T> {
   final T width;
   final T height;
 
-  const Rectangle(this.left, this.top, this.width, this.height);
+  const Rectangle(left, top, width, height)
+      : left = (width >= 0) ? left : left + width,
+        width = (width >= 0) ? width : -width,
+        top = (height >= 0) ? top : top + height,
+        height = (height >= 0) ? height : -height;

[srdjan, 2014/01/28 16:47:41]

This looks very unreadable to me (same names for arguments and instance
variables). Shouldn't the arguments be typed? (see style guide)

[Lasse Reichstein Nielsen, 2014/01/28 17:49:59]

Yes, they should be typed "T", and I should write "this.left" on the left-hand
side of the '='.

 
   factory Rectangle.fromPoints(Point<T> a, Point<T> b) {
     T left = min(a.x, b.x);
@@ -145,14 +153,18 @@ class Rectangle<T extends num> extends _RectangleBase<T> {
  * A class for representing two-dimensional axis-aligned rectangles with mutable
  * properties.
  */
-class MutableRectangle<T extends num>  extends _RectangleBase<T>
-    implements Rectangle<T> {
+class MutableRectangle<T extends num> extends _RectangleBase<T>
+                                      implements Rectangle<T> {
   T left;
   T top;
   T width;
   T height;
 
-  MutableRectangle(this.left, this.top, this.width, this.height);
+  MutableRectangle(left, top, width, height)
+      : left = (width >= 0) ? left : left + width,
+        width = (width >= 0) ? width : -width,
+        top = (height >= 0) ? top : top + height,
+        height = (height >= 0) ? height : -height;
 
   factory MutableRectangle.fromPoints(Point<T> a, Point<T> b) {
     T left = min(a.x, b.x);