Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_wrapping_function_frog_test.dart

 // Copyright (c) 2012, 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";
-
 typedef void Callback0();
 typedef void Callback1(arg1);
 typedef void Callback2(arg1, arg2);
 
 class A native "*A" {
   foo0(Callback0 closure) native;
   foo1(Callback1 closure, arg1) native;
   foo2(Callback2 closure, arg1, arg2) native;
 }
 
@@ skipping 15 matching lines @@
   var a = makeA();
   Expect.equals(42, a.foo0(() => 42));
   Expect.equals(43, a.foo1((arg1) => arg1, 43));
   Expect.equals(44, a.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
 
   A aa = a;
   Expect.equals(42, aa.foo0(() => 42));
   Expect.equals(43, aa.foo1((arg1) => arg1, 43));
   Expect.equals(44, aa.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
 }