Revert "Remove Expect from core library."

tests/language/expect_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.
 // Testing the Expect class.
 
-import "package:expect/expect.dart";
-
 class ExpectTest {
 
   static testEquals(a) {
     try {
       Expect.equals("AB", a, "within testEquals");
     } on Exception catch (msg) {
       print(msg);
       return;
     }
     Expect.equals("AB", "${a}B");
@@ skipping 52 matching lines @@
     testIsFalse(0);
     testIdentical("A");
     testFail();
   }
 
 }
 
 main() {
   ExpectTest.testMain();
 }