Revert "Remove Expect from core library."

tests/language/optimized_string_charat_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 optimized [] on strings.
 
 var a = "abc";
 var b = "øbc";
 var c = new String.fromCharCodes([123, 456, 789]);
 
 test_charat(s, i) {
   return s[i];
 }
 
@@ skipping 41 matching lines @@
 
   Expect.equals(new String.fromCharCodes([123]), test_charat(c, 0));
   for (var i=0; i<10000; i++) test_charat(c, 0);
   Expect.equals(new String.fromCharCodes([123]), test_charat(c, 0));
   Expect.equals(new String.fromCharCodes([456]), test_charat(c, 1));
   Expect.equals(new String.fromCharCodes([789]), test_charat(c, 2));
   Expect.throws(() => test_charat(c, 3));
 
 
 }