Revert "Remove Expect from core library."

tests/standalone/io/process_path_environment_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 that the executable is looked up on the user's PATH when spawning a
 // process and environment variables are passed in.
 
-import "package:expect/expect.dart";
 import "dart:io";
 
 main() {
   // Pick an app that we expect to be on the PATH that returns 0 when run with
   // no arguments.
   var executable = 'true';
   var args = [];
   if (Platform.operatingSystem == 'windows') {
     executable = 'cmd.exe';
     args = ['/C', 'echo', '"ok"'];
   }
 
   var options = new ProcessOptions();
   options.environment = new Map.from(Platform.environment);
   options.environment['whatever'] = 'something';
 
   Process.run(executable, args, options).then((result) {
     Expect.equals(0, result.exitCode);
   });
 }