Revert "Remove Expect from core library."

tests/compiler/dart2js/builtin_equals_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.
 
-import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const String TEST = r"""
 foo() {
   String s = new Object().toString();
   Object o = new Object().toString();
   return s == 'foo'
     && s == null
     && null == s
     && null == o;
 }
 """;
 
 main() {
   String generated = compile(TEST, entry: 'foo', enableTypeAssertions: true);
   Expect.isTrue(!generated.contains('eqB'));
 
   RegExp regexp = new RegExp('==');
   Iterator<Match> matches = regexp.allMatches(generated).iterator;
   checkNumberOfMatches(matches, 4);
 }