Revert "Remove Expect from core library."

tests/language/mixin_this_use_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.
 
 // Test that [:this:] in a class A used as a mixin in class D knows it can be an
 // instance of D.
 
-import "package:expect/expect.dart";
-
 class A {
   foo() => bar(); // Implicit use of [:this:]
   bar() => 42;
 }
 
 class B {}
 
 typedef C = B with A;
 
 class D extends C {
   bar() => 54;
 }
 
 class E extends A {
   bar() => 68;
 }
 
 main() {
   Expect.equals(54, new D().foo());
   Expect.equals(68, new E().foo());
 }