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();
   testToNativePath();
   testCanonicalize();
   testJoinAppend();
   testRelativeTo();
   testWindowsShare();
@@ skipping 314 matching lines @@
   Expect.equals('../../c', CPath.relativeTo(cPath).toString());
   Expect.equals('../b/d', cPath.relativeTo(CPath).toString());
   Expect.equals('.', DPath.relativeTo(DPath).toString());
   Expect.equals('.', NoPath.relativeTo(NoPath).toString());
   Expect.equals('.', C2Path.relativeTo(cPath).toString());
   Expect.equals('..', C3Path.relativeTo(cPath).toString());
   Expect.equals('d', cPath.relativeTo(C3Path).toString());
   Expect.equals('a/b/d', cPath.relativeTo(C4Path).toString());
   Expect.equals('../../../', C4Path.relativeTo(cPath).toString());
 }