Make Rectangle and MutableRectangle constructors handle negative lengths.

tests/lib/math/rectangle_test.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.
 
 library rect_test;
 
 import 'dart:math';
 import 'package:unittest/unittest.dart';
 
 main() {
@@ skipping 142 matching lines @@
     var bignum = 0x80000000000008 + 0.0;
     var r1 = new Rectangle(bignum, bignum, 1.0, 1.0);
     var r2 = new Rectangle(bignum, bignum, 2.0, 2.0);
     expect(r1, r2);
     expect(r1.hashCode, r2.hashCode);
     expect(r1.right, r2.right);
     expect(r1.bottom, r2.bottom);
     expect(r1.width, 1.0);
     expect(r2.width, 2.0);
   });
+
+  test('negative lengths', () {
+    // Constructor allows negative lengths, but clamps them to zero.
+    expect(new Rectangle(4, 4, -2, -2), new Rectangle(4, 4, 0, 0));
+    expect(new MutableRectangle(4, 4, -2, -2), new Rectangle(4, 4, 0, 0));
+
+    // Setters clamp negative lengths to zero.
+    var r = new MutableRectangle(0, 0, 1, 1);
+    r.width = -1;
+    r.height = -1;
+    expect(r, new Rectangle(0, 0, 0, 0));
+
+    // Test that doubles are clamped to double zero.
+    r = new Rectangle(1.5, 1.5, -2.5, -2.5);
+    expect(identical(r.width, 0.0), isTrue);
+    expect(identical(r.height, 0.0), isTrue);
+  });
 }
+