Revert "Remove Expect from core library."

tests/language/return_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.
 
-import "package:expect/expect.dart";
-
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch (e) {
     return true;
   }
 }
 
 int returnString1() => 's';
 void returnNull() => null;
 void returnString2() => 's';
 
 main() {
   if (isCheckedMode()) {
     Expect.throws(returnString1, (e) => e is TypeError);
     Expect.throws(returnString2, (e) => e is TypeError);
     returnNull();
   } else {
     returnString1();
     returnNull();
     returnString2();
   }
 }