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() {
   Rectangle createRectangle(List<num> a) {
     return a != null ? new Rectangle(a[0], a[1], a[2] - a[0], a[3] - a[1])
         : null;
   }
 
   test('construction', () {
     var r0 = new Rectangle(10, 20, 30, 40);
     expect(r0.toString(), 'Rectangle (10, 20) 30 x 40');
     expect(r0.right, 40);
     expect(r0.bottom, 60);
 
     var r1 = new Rectangle.fromPoints(r0.topLeft, r0.bottomRight);
     expect(r1, r0);
 
     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', () {
     var tests = [
         [[10, 10, 20, 20], [15, 15, 25, 25], [15, 15, 20, 20]],
         [[10, 10, 20, 20], [20, 0, 30, 10], [20, 10, 20, 10]],
         [[0, 0, 1, 1], [10, 11, 12, 13], null],
         [[11, 12, 98, 99], [22, 23, 34, 35], [22, 23, 34, 35]]];
 
     for (var test in tests) {
@@ skipping 85 matching lines @@
 
   test('hashCode', () {
     var a = new Rectangle(0, 1, 2, 3);
     var b = new Rectangle(0, 1, 2, 3);
     expect(a.hashCode, b.hashCode);
 
     var c = new Rectangle(1, 0, 2, 3);
     expect(a.hashCode == c.hashCode, isFalse);
   });
 }