Remove Expect from core library.

tests/language/generic_native_test.dart

 // Copyright (c) 2013, 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 is-tests with type variables on native subclasses.
 
+import "package:expect/expect.dart";
+
 class A<T> {
   foo(o) => o is T;
 }
 
 class B {}
 
 class C {}
 
 main() {
   Expect.isTrue(new A<Iterable<B>>().foo(new List<B>()));
   Expect.isFalse(new A<Iterable<C>>().foo(new List<B>()));
 
   Expect.isTrue(new A<Collection<B>>().foo(new List<B>()));
   Expect.isFalse(new A<Collection<C>>().foo(new List<B>()));
 
   Expect.isTrue(new A<Pattern>().foo('hest'));
 
   Expect.isTrue(new A<Comparable<String>>().foo('hest'));
   Expect.isFalse(new A<Comparable<C>>().foo('hest'));
 }