Revert "Remove Expect from core library."

tests/language/exception_test.dart

 // Copyright (c) 2011, 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 ExceptionTest {
   static testMain() {
     int i = 0;
     try {
       throw "Hello";
     } on String catch (s) {
       print(s);
       i += 10;
     }
 
@@ skipping 16 matching lines @@
     } catch (x) {
       correctCatch = false;
     }
     Expect.isTrue(correctCatch);
   }
 }
 
 main() {
   ExceptionTest.testMain();
 }