Revert "Remove Expect from core library."

tests/language/library5_test.dart

 // Copyright (c) 2011, 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 Library5Test.dart;
-import "package:expect/expect.dart";
 import "library5a.dart" as lib5a;
 import "library5b.dart" as lib5b;
 
 int foo(lib5a.Fun f) {
   return f();
 }
 
 int bar(lib5b.Fun f) {
   return f(42);
 }
 
 main() {
   Expect.equals(41, foo(() => 41));
   Expect.equals(42, bar((x) => x));
 }