Revert "Remove Expect from core library."

tests/compiler/dart2js/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";
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 class A {}
 bool foo(bar) {
   var x = new A();
   var y = new A();
   return identical(x, y);
 }
 """;
 
 main() {
   String generated = compile(TEST_ONE, entry: 'foo');
 
   // Check that no boolify code is generated.
   RegExp regexp = new RegExp("=== true");
   Iterator matches = regexp.allMatches(generated).iterator;
   Expect.isFalse(matches.hasNext);
 
   regexp = new RegExp("===");
   matches = regexp.allMatches(generated).iterator;
   Expect.isTrue(matches.moveNext());
   Expect.isFalse(matches.moveNext());
 }