Revert "Remove Expect from core library."

tests/language/first_class_types_libraries_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.
 
 library firstClassLibrariestest;
-import "package:expect/expect.dart";
 import 'first_class_types_lib1.dart' as lib1;
 import 'first_class_types_lib2.dart' as lib2;
 
 class C<X> {}
 
 sameType(a, b) {
   Expect.equals(a.runtimeType, b.runtimeType);
 }
 
 differentType(a, b) {
   Expect.notEquals(a.runtimeType, b.runtimeType);
 }
 
 main() {
   sameType(new lib1.A(), new lib1.A());
   differentType(new lib1.A(), new lib2.A());
   differentType(new C<lib1.A>(), new C<lib2.A>());
 }