Revert "Remove Expect from core library."

tests/language/canonical_const_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 initializers of static const fields are compile time constants.
 
-import "package:expect/expect.dart";
-
 class CanonicalConstTest {
   static const A = const C1();
   static const B = const C2();
 
   static testMain() {
     Expect.identical(null, null);
     Expect.isFalse(identical(null, 0));
     Expect.identical(1, 1);
     Expect.isFalse(identical(1, 2));
     Expect.identical(true, true);
@@ skipping 19 matching lines @@
   const C1();
 }
 
 class C2 extends C1 {
   const C2() : super();
 }
 
 main() {
   CanonicalConstTest.testMain();
 }