Make Rectangle and MutableRectangle constructors handle negative lengths.

tests/lib/math/rectangle_test.dart

Index: tests/lib/math/rectangle_test.dart
diff --git a/tests/lib/math/rectangle_test.dart b/tests/lib/math/rectangle_test.dart
index 9e1c47957ef650d9c0ae76b2c2aa09286872d95a..2135208c680bd84165347d12e038cebab2f777fa 100644
--- a/tests/lib/math/rectangle_test.dart
+++ b/tests/lib/math/rectangle_test.dart
@@ -24,6 +24,9 @@ main() {
 
     var r2 = new Rectangle.fromPoints(r0.bottomRight, r0.topLeft);
     expect(r2, r0);
+
+    var r3 = new Rectangle(40, 60, -30, -40);
+    expect(r3, r0);
   });
 
   test('intersection', () {