Remove Expect from core library.

tests/compiler/dart2js_native/native_method_rename1_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.
 
 // Test the feature where the native string declares the native method's name.
 
+import "package:expect/expect.dart";
 import 'dart:_js_helper' show JSName;
 
 class A native "*A" {
   @JSName('fooA')
   int foo() native;
 
   @JSName('barA')
   int bar() native;
 
   @JSName('bazA')
@@ skipping 52 matching lines @@
 expectNoSuchMethod(action, note) {
   bool caught = false;
   try {
     action();
   } catch (ex) {
     caught = true;
     Expect.isTrue(ex is NoSuchMethodError, note);
   }
   Expect.isTrue(caught, note);
 }