Revert "Remove Expect from core library."

tests/corelib/null_nosuchmethod_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.
 
-import "package:expect/expect.dart";
-
 // Test that Null's noSuchMethod can be closurized and called directly.
 
 main() {
   var x;
   // Non-existing method calls noSuchMethod.
   Expect.throws(() => x.foo(),
                 (e) => e is NoSuchMethodError);
 
   // Calling noSuchMethod directly.
   Expect.throws(() => x.noSuchMethod("foo", []),
                 (e) => e is NoSuchMethodError);
 
   // Closurizing noSuchMethod and calling it.
   var nsm = x.noSuchMethod;
   Expect.notEquals(null, nsm);
   Expect.throws(() => nsm("foo", []),
                 (e) => e is NoSuchMethodError);
 }