Remove Expect from core library.

tests/language/mixin_extends_is_test.dart

 // Copyright (c) 2013, 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";
+
 class S { }
 class M1 { }
 class M2 { }
 
 class C extends S with M1 { }
 class D extends S with M1, M2 { }
 class E extends S with M2, M1 { }
 class F extends E { }
 
 class C_ extends S with M1 { }
@@ skipping 53 matching lines @@
   Expect.isFalse(e is C_);
   Expect.isFalse(e is D_);
   Expect.isFalse(e is E_);
   Expect.isFalse(e is F_);
 
   Expect.isFalse(f is C_);
   Expect.isFalse(f is D_);
   Expect.isFalse(f is E_);
   Expect.isFalse(f is F_);
 }