Revert "Remove Expect from core library."

tests/corelib/string_pattern_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 for testing String.allMatches.
 
-import "package:expect/expect.dart";
-
 String str = "this is a string with hello here and hello there";
 
 main() {
   testNoMatch();
   testOneMatch();
   testTwoMatches();
   testEmptyPattern();
   testEmptyString();
   testEmptyPatternAndString();
 }
@@ skipping 52 matching lines @@
   Iterable<Match> matches = pattern.allMatches(str);
   Expect.isFalse(matches.iterator.moveNext());
 }
 
 testEmptyPatternAndString() {
   String pattern = "";
   String str = "";
   Iterable<Match> matches = pattern.allMatches(str);
   Expect.isTrue(matches.iterator.moveNext());
 }