Revert "Remove Expect from core library."

tests/language/code_after_try_is_executed_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.
 // Test that the runtime still runs the code after a try/catch. The
 // test cannot use Expect.throws, because Expect.throws uses the same
 // pattern.
 
-import "package:expect/expect.dart";
-
 main() {
   var exception;
   try {
     throw 'foo';
   } on String catch (ex) {
     exception = ex;
   }
   Expect.isTrue(exception is String);
   throw 'foo'; /// 01: runtime error
 }