Revert "Remove Expect from core library."

tests/language/stack_overflow_stacktrace_test.dart

 // Copyright (c) 2013, 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 program testing stack overflow.
 
-import "package:expect/expect.dart";
-
 class StackOverflowTest {
 
   static void curseTheRecurse(a, b, c) {
     curseTheRecurse(b, c, a);
   }
 
   static void testMain() {
     bool exceptionCaught = false;
     try {
       curseTheRecurse(1, 2, 3);
     } on StackOverflowError catch (e, stacktrace) {
       String s = stacktrace.toString();
       Expect.equals(-1, s.indexOf("-1:-1"));
       exceptionCaught = true;
     }
     Expect.equals(true, exceptionCaught);
   }
 }
 
 main() {
   StackOverflowTest.testMain();
 }