Revert "Remove Expect from core library."

tests/language/optimized_string_charcodeat_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.
 
 // Test optimized CodeUnitAt and array access.
 
-import "package:expect/expect.dart";
-
-
 String one_byte = "hest";
 String two_byte = "høns";
 
 int testOneByteCodeUnitAt(String x, int j) {
   int test() {
     return x.codeUnitAt(j);
   }
   for (int i = 0; i < 10000; i++) test();
   return test();
 }
@@ skipping 51 matching lines @@
     Expect.equals(248, testConstantStringCodeUnitAt(1));
     Expect.equals(101, testConstantIndexCodeUnitAt(one_byte));
   }
   for (int j = 0; j < 2000; j++) {
     Expect.equals(436, testOneByteCodeUnitAtInLoop(one_byte));
     Expect.equals(577, testTwoByteCodeUnitAtInLoop(two_byte));
   }
   Expect.throws(() => testOneByteCodeUnitAtInLoop(123));
   Expect.throws(() => testTwoByteCodeUnitAtInLoop(123));
 }