Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/fields_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";
-
 class A {
   A() {
   }
   int x;
 
   foo() {
     x = 42;
     Expect.equals(42, x);
     x = 0;
     Expect.equals(0, x);
   }
 }
 
 class B extends A {
 }
 
 main() {
   A a = new A();
   a.foo();
   Expect.equals(0, a.x);
   a.x = 4;
   Expect.equals(4, a.x);
   a.x += 1;
   Expect.equals(5, a.x);
 
   B b = new B();
   b.foo();
   Expect.equals(0, b.x);
 }