Revert "Remove Expect from core library."

tests/compiler/dart2js_foreign/native_null_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.
 
 // Test that parameters in native methods are not mangled. This test is needed
 // until we change all libraries to using the JS foreign element.
 
-import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 @Native("*A")
 class A {
   @Native("return null;")
   returnNull();
   @Native("return undefined;")
   returnUndefined();
   @Native("return '';")
   returnEmptyString();
@@ skipping 16 matching lines @@
   Expect.equals(null, a.returnNull());
   Expect.equals(null, a.returnUndefined());
 
   Expect.equals('', a.returnEmptyString());
   Expect.isTrue(a.returnEmptyString().isEmpty);
   Expect.isTrue(a.returnEmptyString() is String);
 
   Expect.isTrue(a.returnZero() is int);
   Expect.equals(0, a.returnZero());
 }