Revert "Remove Expect from core library."

tests/language/static_postfix_operator_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";
-
 int a = 0;
 int b = 0;
 
 withTryFinally() {
   bool inIt = false;
   // Do a try/finally to potentially force a non-optimizing compiler.
   try {
     if (a++ == 0) {
       inIt = true;
     }
   } finally {
   }
   Expect.isTrue(inIt);
 }
 
 withoutTryFinally() {
   bool inIt = false;
   if (b++ == 0) {
     inIt = true;
   }
   Expect.isTrue(inIt);
 }
 
 main() {
   withTryFinally();
   withoutTryFinally();
 }