Remove Expect from core library.

tests/compiler/dart2js_foreign/native_exceptions1_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 hidden native exception classes can be marked as existing.
 //
 // To infer which native hidden types exist, we need
 //   (1) return types of native methods and getters
 //   (2) argument types of callbacks
 //   (3) exceptions thrown by the operation.
 //
 // (1) and (2) can be achieved by having nicely typed native methods, but there
 // is no place in the Dart language to communicate (3).  So we use the following
 // fake body technique.
 
+import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 // The exception type.
 @Native("*E")
 class E {
   @native E._used();  // Bogus native constructor, called only from fake body.
 
   final int code;
 }
 
@@ skipping 69 matching lines @@
   threw = false;
   try {
     var x = aa.op(51);
   } on E catch (e) {
     threw = true;
     Expect.equals(100, e.code);
     Expect.isTrue(e is E);
   }
   Expect.isTrue(threw);
 }