Revert "Remove Expect from core library."

tests/standalone/io/test_extension_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.
 //
 // Dart test program for testing native extensions.
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
 Future copyFileToDirectory(Path file, Path directory) {
   String src = file.toNativePath();
   String dst = directory.toNativePath();
   switch (Platform.operatingSystem) {
     case 'linux':
     case 'macos':
@@ skipping 39 matching lines @@
   }).then((_) {
     Path script = testDirectory.append('test_extension_tester.dart');
     return Process.run(options.executable, [script.toNativePath()]);
   })..then((ProcessResult result) {
     Expect.equals(0, result.exitCode);
     tempDirectory.deleteSync(recursive: true);
   })..catchError((_) {
     tempDirectory.deleteSync(recursive: true);
   });
 }