Revert "Remove Expect from core library."

tests/language/savannah_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.
 // Dart test using an identity hash.
 
-import "package:expect/expect.dart";
-
 abstract class BigGame {
   String get name;
 }
 
 // Giraffe overrides hashCode and provides its own identity hash.
 class Giraffe implements BigGame {
   final String name;
   final int identityHash_;
 
   Giraffe(this.name) : identityHash_ = nextId() {}
@@ skipping 55 matching lines @@
     print("zebra2: ${savannah[zebra2]}");
     Expect.equals("Paolo", savannah[zebra1]);
     Expect.equals("Zeeta", savannah[zebra2]);
   }
 
 }
 
 main() {
   SavannahTest.testMain();
 }