Add package with iterable and stream "zipping" functionality.

pkg/sequence_zip/lib/sequence_zip.dart

Index: pkg/sequence_zip/lib/sequence_zip.dart
diff --git a/tests/language/extends_test_lib.dart b/pkg/sequence_zip/lib/sequence_zip.dart
similarity index 74%
copy from tests/language/extends_test_lib.dart
copy to pkg/sequence_zip/lib/sequence_zip.dart
index d62ca8f7da480b38ac34aed6411d25acba5ae339..e8331ac4adb7a6ef0b38443835b5c36fe8eba911 100644
--- a/tests/language/extends_test_lib.dart
+++ b/pkg/sequence_zip/lib/sequence_zip.dart
@@ -2,8 +2,7 @@
 // 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.
 
-library ExtendsTestLib;
+library sequence_zip;
 
-class A {
-  var y = "class A from library";
-}
+export "iterable_zip.dart";
+export "stream_zip.dart";