Revert "Remove Expect from core library."

tests/language/first_class_types_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";
-
 class C<T> {}
 
 sameType(a, b) {
   Expect.equals(a.runtimeType, b.runtimeType);
 }
 
 differentType(a, b) {
   Expect.isFalse(a.runtimeType == b.runtimeType);
 }
 
@@ skipping 18 matching lines @@
   var l = [1, 2, 3];
   var m = {'a': 1, 'b': 2};
   sameType([], l);
   sameType({}, m);
 
   // Test parameterized lists.
   sameType(new List<int>(), new List<int>());
   differentType(new List<int>(), new List<num>());
   differentType(new List<int>(), new List());
 }