Revert "Remove Expect from core library."

tests/language/dynamic_field_test.dart

 // Copyright (c) 2012, 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 ensures that fields can be accessed dynamically.
 
-import "package:expect/expect.dart";
-
 class A extends C {
   var a;
   var b;
 }
 
 class C {
   foo() {
     print(a); /// static type warning
     return a; /// static type warning
   }
   bar() {
     print(b.a); /// static type warning
     return b.a; /// static type warning
   }
 }
 
 main() {
   var a = new A();
   a.a = 1;
   a.b = a;
   Expect.equals(1, a.foo());
   Expect.equals(1, a.bar());
 }