Revert "Remove Expect from core library."

tests/language/is_object_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 program for the "is" type test operator.
 
-import "package:expect/expect.dart";
-
 testTryCatch(x) {
   try {
     throw x;
     Expect.fail("Exception '$x' should've been thrown");
   } on Object catch (obj) {
     Expect.equals(obj, x);
   }
 }
 
 main() {
@@ skipping 20 matching lines @@
   Expect.isTrue(testEval(false) is Object);
   Expect.equals(3, evalCount);
   Expect.isTrue(testEval(null) is Object);
   Expect.equals(4, evalCount);
 
   // Verify that these objects are catchable
   testTryCatch(444);
   testTryCatch('abc');
   testTryCatch(true);
 }