Revert "Remove Expect from core library."

tests/standalone/io/path_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.
 //
 // Test the Path class in dart:io.
 
-import "package:expect/expect.dart";
 import "dart:io";
 
 void main() {
   testBaseFunctions();
   testRaw();
   testCanonicalize();
   testJoinAppend();
   testRelativeTo();
   testWindowsShare();
 }
@@ skipping 277 matching lines @@
   Expect.isTrue(canonical.toString().startsWith('/share/a'));
   Expect.isTrue(canonical.toNativePath().startsWith(r'\\share\a'));
   Expect.listEquals(['share', 'a', 'c'], canonical.segments());
   var appended = canonical.append('d');
   Expect.isTrue(appended.isAbsolute);
   Expect.isTrue(appended.isWindowsShare);
   var directoryPath = canonical.directoryPath;
   Expect.isTrue(directoryPath.isAbsolute);
   Expect.isTrue(directoryPath.isWindowsShare);
 }