Remove Expect from core library.

tests/compiler/dart2js_native/native_mixin_field_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 with fields
 // as mixins.
 
 class A native "*A" {
   var foo;
 }
 
 class B extends A with M1, M2 native "*B" {
   var bar;
 }
@@ skipping 36 matching lines @@
   Expect.throws(() => m1.bar, (e) => e is NoSuchMethodError);
   Expect.isNull(m1.baz);
   Expect.throws(() => m1.buz, (e) => e is NoSuchMethodError);
 
   M2 m2 = new M2();
   Expect.throws(() => m2.foo, (e) => e is NoSuchMethodError);
   Expect.isNull(m2.bar);
   Expect.throws(() => m2.baz, (e) => e is NoSuchMethodError);
   Expect.isNull(m2.buz);
 }