add enumerate to the default filter set

pkg/polymer_expressions/lib/src/globals.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.
+
+/**
+ * Contains functions that are included by default in [PolymerExpressions].
+ *
+ *   - [enumerate]: a convenient way to iterate over items and the indexes.
+ */
+library polymer_expressions.src.globals;
+
+/**
+ * Returns an [Iterable] of [IndexedValue]s where the nth value holds the nth
+ * element of [iterable] and its index.
+ */
+Iterable<IndexedValue> enumerate(Iterable iterable) {
+  int i = 0;
+  return iterable.map((e) => new IndexedValue(i++, e));
+}
+
+class IndexedValue<V> {
+  final int index;
+  final V value;
+
+  IndexedValue(this.index, this.value);
+}