Add package with iterable and stream "zipping" functionality.

pkg/sequence_zip/lib/iterable_zip.dart

Index: pkg/sequence_zip/lib/iterable_zip.dart
diff --git a/pkg/sequence_zip/lib/iterable_zip.dart b/pkg/sequence_zip/lib/iterable_zip.dart
new file mode 100644
index 0000000000000000000000000000000000000000..90c4c4ff5dce64e33b08461c3cd4c1c8dd04c0da
--- /dev/null
+++ b/pkg/sequence_zip/lib/iterable_zip.dart
@@ -0,0 +1,56 @@
+// 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.
+
+library iterable_zip;
+
+import "dart:collection";
+
+/**
+ * Iterable that iterates over lists of values from other iterables.
+ *
+ * When [iterator] is read, an [Iterator] is created for each [Iterable] in
+ * the [Iterable] passed to the constructor.
+ *
+ * As long as all these iterators have a next value, those next values are
+ * combined into a single list, which becomes the next value of this
+ * [Iterable]'s [Iterator]. As soon as any of the iterators run out,
+ * the zipped iterator also stops.
+ */
+class IterableZip extends IterableBase<List> {
+  final Iterable<Iterable> _iterables;
+  IterableZip(Iterable<Iterable> iterables)
+      : this._iterables = iterables;
+
+  /**
+   * Returns an iterator that combines values of the iterables' iterators
+   * as long as they all have values.
+   */
+  Iterator<List> get iterator {
+    List iterators = _iterables.map((x) => x.iterator).toList(growable: false);
+    // TODO(lrn): Return an empty iterator directly if iterators is empty?
+    return new _IteratorZip(iterators);
+  }
+}
+
+class _IteratorZip implements Iterator<List> {
+  final List<Iterator> _iterators;
+  List _current;
+  _IteratorZip(List iterators) : _iterators = iterators;
+  bool moveNext() {
+    if (_iterators.isEmpty) return false;
+    for (int i = 0; i < _iterators.length; i++) {
+      if (!_iterators[i].moveNext()) {
+        _current = null;
+        return false;
+      }
+    }
+    _current = new List(_iterators.length);
+    for (int i = 0; i < _iterators.length; i++) {
+      _current[i] = _iterators[i].current;
+    }
+    return true;
+  }
+
+  List get current => _current;
+}