Revert "Remove Expect from core library."

tests/compiler/dart2js/constant_folding_test.dart

 // Copyright (c) 2012, 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.
 // Test constant folding on numbers.
 
-import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const String NUMBER_FOLDING = """
 void main() {
   var a = 4;
   var b = 3;
   print(a + b);
 }
 """;
 
@@ skipping 26 matching lines @@
 
   regexp = new RegExp(r'null == b');
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = new RegExp(r'4 === c');
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = new RegExp('"foo" === d');
   Expect.isTrue(regexp.hasMatch(generated));
 }