Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/static_field_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 {
   static int b;
 
   setA(val) {
     b = val;
   }
 
   bar() {
     return b;
   }
 
   bar2() {
     return A.b;
   }
 }
 
 main() {
   A a = new A();
   a.setA(42);
   Expect.equals(42, a.bar());
   Expect.equals(42, a.bar2());
 }