Revert "Remove Expect from core library."

tests/language/stack_trace_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.
 // Dart test program for testing throw statement
 
-import "package:expect/expect.dart";
-
 class MyException {
   const MyException(String message) : message_ = message;
   final String message_;
 }
 
 class Helper {
   static int f1(int i) {
     try {
       i = func();
       i = 10;
@@ skipping 61 matching lines @@
 
 class StackTraceTest {
   static testMain() {
     Expect.equals(850, Helper.f1(1));
   }
 }
 
 main() {
   StackTraceTest.testMain();
 }