Revert "Remove Expect from core library."

tests/language/switch_fallthru_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.
 // Check that FallThroughError is thrown if switch clause does not terminate.
 
-import "package:expect/expect.dart";
-
 class SwitchFallthruTest {
   static String test(int n) {
     String result = "foo";
     switch (n) {
       case 0:
         result = "zero";
         break;
       case 1:
         result = "one";
         // fall-through, throw implicit FallThroughError here.
@@ skipping 14 matching lines @@
     }
     Expect.equals(true, fallthroughCaught);
     Expect.equals("nine", test(9));
     Expect.equals("foo", test(99));
   }
 }
 
 main() {
   SwitchFallthruTest.testMain();
 }