Remove Expect from core library.

tests/language/many_overridden_no_such_method_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.
 
 library ManyOverriddenNoSuchMethodTest.dart;
+import "package:expect/expect.dart";
 part "overridden_no_such_method.dart";
 
 class ManyOverriddenNoSuchMethodTest {
   static testMain() {
     for (int i = 0; i < 5000; i++) {
       OverriddenNoSuchMethod.testMain();
     }
   }
 }
 
 main() {
   ManyOverriddenNoSuchMethodTest.testMain();
 }