Revert "Remove Expect from core library."

tests/language/closure_type_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.
 // Dart test for a closure result type test that cannot be eliminated at compile
 // time.
 
 library closure_type_test;
-import "package:expect/expect.dart";
 import 'dart:math' as math;
 
 class Math {
   static int sqrt(x) => math.sqrt(x);
 }
 
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
@@ skipping 18 matching lines @@
   } on TypeError catch (error) {
     got_type_error = true;
   }
   // Type error expected in checked mode only.
   Expect.isTrue(got_type_error == isCheckedMode());
 }
 
 root(x) => Math.sqrt(x);
 
 main() => test(root, 4);