Remove Expect from core library.

tests/compiler/dart2js/boolify_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.
 
 library boolified_operator_test;
+import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const String TEST = r"""
 foo() {
   var a = foo();
   if (!a) return 1;
   return 2;
 }
 """;
 
 main() {
   String generated = compile(TEST, entry: 'foo');
   Expect.isTrue(generated.contains('foo() !== true)'));
 }