Revert "Remove Expect from core library."

tests/standalone/io/regress_7679_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 'dart:io';
 
 main() {
   Directory temp = new Directory('').createTempSync();
   File script = new File('${temp.path}/script.dart');
   script.writeAsStringSync("""
 import 'dart:io';
 
-class Expect {
-  static void isTrue(var x) {
-    if (!identical(x, true)) {
-      throw new Error("Not identical");
-    }
-  }
-}
-
 main() {
   Directory d = new Directory('a');
   d.create(recursive: true).then((_) {
     d.exists().then((result) {
       Expect.isTrue(result);
       d = new Directory('b/c/d');
       d.create(recursive: true).then((_) {
         d.exists().then((result) {
           Expect.isTrue(result);
         });
       });
     });
   });
 }
 """);
   ProcessOptions options = new ProcessOptions();
   options.workingDirectory = temp.path;
   String executable = new File(new Options().executable).fullPathSync();
   Process.run(executable, ['script.dart'], options).then((result) {
     temp.deleteSync(recursive: true);
     Expect.equals(0, result.exitCode);
   });
 }