Revert "Remove Expect from core library."

tests/corelib/iterable_expand_test.dart

 // Copyright (c) 2013, 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";
-
 main() {
   test(expectation, iterable) {
     Expect.listEquals(expectation, iterable.toList());
   }
 
   // Function not called on empty iterable.
   test([], [].expand((x) { throw "not called"; }));
 
   // Creating the iterable doesn't call the function.
   [1].expand((x) { throw "not called"; });
@@ skipping 16 matching lines @@
   Iterator it = iterable.iterator;
   Expect.isTrue(it.moveNext());
   Expect.equals(1, it.current);
   Expect.isTrue(it.moveNext());
   Expect.equals(1, it.current);
   Expect.throws(it.moveNext, (e) => e == "FAIL");
   // After throwing, iteration is ended.
   Expect.equals(null, it.current);
   Expect.isFalse(it.moveNext());
 }