Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/string_escape_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";
-
 // Test that string escapes work correctly.
 
 testSingleCharacterEscapes() {
   List/*<String>*/ examples = [
     "\b\f\n\r\t\v",
     '\b\f\n\r\t\v',
     """\b\f\n\r\t\v""",
     '''\b\f\n\r\t\v''',
   ];
   List values = [8, 12, 10, 13, 9, 11];
@@ skipping 111 matching lines @@
   testUEscapes();
   // Test \b, \f, \n, \r, \t, \v.
   testSingleCharacterEscapes();
   // Test all other single character (identity) escaeps.
   testIdentityEscapes();
   // Test that quotes are handled correctly.
   testQuotes();
   // Test that raw strings are raw.
   testRawStrings();
 }