Revert "Remove Expect from core library."

tests/compiler/dart2js/for_in_test.dart

 // Copyright (c) 2013, 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"""
 foo(a) {
   int x = 0;
   for (int i in a) {
     x += i;
   }
   return x;
 }
 """;
 
 const String TEST_TWO = r"""
 foo(a) {
   int x = 0;
   for (int i in a) {
     if (i == 5) continue;
     x += i;
   }
   return x;
 }
 """;
 
 main() {
   String generated = compile(TEST_ONE, entry: 'foo');
   Expect.isTrue(!generated.contains(r'break'));
   generated = compile(TEST_TWO, entry: 'foo');
   Expect.isTrue(generated.contains(r'continue'));
 }