Revert "Remove Expect from core library."

tests/compiler/dart2js/is_inference2_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_IF_BOOL_FIRST_INSTRUCTION = r"""
 negate(x) {
   if (x is bool) return !x;
   return x;
 }
 """;
 
 main() {
   String generated = compile(TEST_IF_BOOL_FIRST_INSTRUCTION, entry: 'negate');
   Expect.isTrue(generated.contains("!"));  // We want to see !x.
   Expect.isFalse(generated.contains("!="));  // And not !== true.
   Expect.isFalse(generated.contains("true"));
   Expect.isFalse(generated.contains("false"));
 }