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..856c1b8047776d589fae1aa90da2b416169e64ca 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', () {
@@ -128,4 +131,19 @@ main() {
     var c = new Rectangle(1, 0, 2, 3);
     expect(a.hashCode == c.hashCode, isFalse);
   });
+
+  test('negative lengths', () {
+    // Constructor allows negative widths, but converts them to positive.
+    expect(new Rectangle(4, 4, -2, -2), new Rectangle(2, 2, 2, 2));
+    expect(new MutableRectangle(4, 4, -2, -2), new Rectangle(2, 2, 2, 2));
+    // Setters do not allow setting negative lengths.
+    var r = new MutableRectangle(0, 0, 1, 1);
+    expect(() { r.width = -1; }, throws);
+    expect(() { r.height = -1; }, throws);
+    // Minus zero isn't negative.
+    r = new MutableRectangle<double>(0.0, 0.0, 1.0, 1.0);
+    r.width = -0.0;
+    r.height = -0.0;
+    expect(r, new Rectangle(0.0, 0.0, 0.0, 0.0));
+  });
 }