Revert "Remove Expect from core library."

tests/language/switch_label_test.dart

 // Copyright (c) 2011, 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.
 // Test switch statement using labels.
 
-import "package:expect/expect.dart";
-
 class Switcher {
 
   Switcher() { }
 
   say1 (sound) {
     var x = 0;
     switch (sound) {
       MOO:
       case "moo":
         x = 100;
@@ skipping 75 matching lines @@
     Expect.equals(300, s.say3("dog", "woof"));
     Expect.equals(400, s.say3("dog", "boj"));   // Ĉu vi parolas Esperanton?
     Expect.equals(400, s.say3("unicorn", ""));  // Still dead.
     Expect.equals(500, s.say3("angry bird", "whoooo"));
   }
 }
 
 main() {
   SwitchLabelTest.testMain();
 }