Revert "Remove Expect from core library."

tests/language/string_interpolate_null_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.
 // Dart test program testing NPE within string interpolation.
 
-import "package:expect/expect.dart";
-
 class A {
   A(String this.name) {}
   String name;
 }
 
 main() {
   A a = new A("Kermit");
   var s = "Hello Mr. ${a.name}";
   Expect.stringEquals("Hello Mr. Kermit", s);
   a = null;
   try {
     s = "Hello Mr. ${a.name}";
   } on NoSuchMethodError catch (e) {
     return;
   }
   Expect.fail("NoSuchMethodError not thrown");
 }