Remove package:unittest from some tests

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';
+import 'package:test/test.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');
@@ skipping 222 matching lines @@
     for (var r in rectangles) {
       expect(r.containsPoint(new Point(0, 1)), false);
       expect(r.containsRectangle(new Rectangle(0, 1, 2, 3)), false);
       expect(r.intersects(new Rectangle(0, 1, 2, 3)), false);
       expect(r.bottom, isNaN);
       expect(r.height, isNaN);
     }
   });
 }