Remove Expect from core library.

tests/compiler/dart2js_foreign/native_call_arity1_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.
 
 // Test that native methods with unnamed* optional arguments are called with the
 // number of arguments in the call site.  This is necessary because native
 // methods can dispatch on the number of arguments.  Passing null or undefined
 // as the last argument is not the same as passing one fewer argument.
 //
 // * Named arguments are passed in the correct position, so require preceding
 // arguments to be passed.
 
+import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 @Native("*A")
 class A {
   @native int foo(int x);
 }
 
 @Native("*B")
 class B {
   @native int foo([x, y, z]);
@@ skipping 57 matching lines @@
   Expect.equals(2, b.foo(y: 20));
   Expect.equals(3, b.foo(z: 30));
   Expect.throws(() => b.foo(10, 20, 30, 40));
 }
 
 main() {
   setup();
   testDynamicContext();
   testStaticContext();
 }