Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_mixin_multiple_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.
 
-import "package:expect/expect.dart";
-
 // Test that native classes can use ordinary Dart classes as mixins.
 
 class A native "*A" {
   foo() => "A-foo";
   baz() => "A-baz";
 }
 
 class B extends A with M1, M2 native "*B" {
   bar() => baz();
 }
@@ skipping 48 matching lines @@
 
   M2 m2 = new M2();
   Expect.equals("M2-foo", m2.foo());
   Expect.throws(() => m2.bar(), (error) => error is NoSuchMethodError);
   Expect.throws(() => m2.baz(), (error) => error is NoSuchMethodError);
   Expect.isFalse(m2 is A);
   Expect.isFalse(m2 is B);
   Expect.isFalse(m2 is M1);
   Expect.isTrue(m2 is M2);
 }