Remove Expect from core library.

tests/language/method_override_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";
+
 // Checks that a method with an instantiated return type can override a method
 // with a generic return type.
 
 typedef V RemoveFunctionType<K, V>(K key);
 
 class MapBase<K, V> implements Map<K, V> {
   K remove(K key) {
     throw 'Must be implemented';
   }
 
@@ skipping 35 matching lines @@
 
   // Perform a few more tests.
   x.Tests();
 
   var m = new MapBase<int, int>();
   Expect.isTrue(m.remove is RemoveFunctionType<int, int>);
 
   // Perform a few more tests.
   m.Tests();
 }