Revert "Remove Expect from core library."

tests/corelib/string_trim_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 StringTrimTest {
   static testMain() {
     Expect.equals("", " ".trim());
     Expect.equals("", "     ".trim());
     var a = "      lots of space on the left";
     Expect.equals("lots of space on the left", a.trim());
     a = "lots of space on the right           ";
     Expect.equals("lots of space on the right", a.trim());
     a = "         lots of space           ";
     Expect.equals("lots of space", a.trim());
     a = "  x  ";
     Expect.equals("x", a.trim());
     Expect.equals("", " \t \n \r ".trim());
   }
 }
 
 main() {
   StringTrimTest.testMain();
 }