Revert "Remove Expect from core library."

tests/language/top_level_in_initializer_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.
 
-import "package:expect/expect.dart";
-
 // Test that constructor initializers can access top level elements.
 
 const topLevelField = 1;
 topLevelMethod() => 1;
 get topLevelGetter { return 1; }
 
 class Foo {
   Foo.one() : x = topLevelField;
   Foo.second() : x = topLevelMethod;
   Foo.third() : x = topLevelGetter;
   var x;
 }
 
 main() {
   Expect.equals(topLevelField, new Foo.one().x);
   Expect.equals(topLevelMethod(), new Foo.second().x());
   Expect.equals(topLevelGetter, new Foo.third().x);
 }