Revert "Remove Expect from core library."

tests/language/exception_identity_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.
 
-import "package:expect/expect.dart";
-
 // Test that an object when thrown stays the same.
 
 class A {
   A();
 }
 
 check(exception) {
   try {
     throw exception;
   } catch (e) {
     Expect.equals(exception, e);
   }
 }
 
 main() {
   check("str");
   check(new A());
   check(1);
   check(1.2);
 }