Remove Expect from core library.

tests/compiler/dart2js_native/native_named_constructors3_frog_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.
 
+import "package:expect/expect.dart";
+
 // Hidden native class with factory constructors and NO static methods.
 // Regression test.
 
 
 class A native "*A" {
 
   // No static methods in this class.
 
   factory A(int len) => makeA(len);
 
@@ skipping 17 matching lines @@
 main() {
   setup();
   var a1 = new A(100);
   var a2 = new A.fromString('Hello');
   var a3 = new A.nativeConstructor();
 
   Expect.equals(100, a1.foo());
   Expect.equals(5, a2.foo());
   Expect.equals(102, a3.foo());
 }