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.
+ */
+// Code from https://github.com/google/quiver-dart/commit/52edc4baf37e99ff6a8f99c648b29b135fc0b880
+library polymer_expressions.src.globals;
+
+import 'dart:collection';
+
+/**
+ * Returns an [Iterable] of [IndexedValue]s where the nth value holds the nth
+ * element of [iterable] and its index.
+ */
+Iterable<IndexedValue> enumerate(Iterable iterable) =>
+    new EnumerateIterable(iterable);
+
+class IndexedValue<V> {
+  final int index;
+  final V value;
+
+  IndexedValue(this.index, this.value);
+}
+
+
+/**
+ * An [Iterable] of [IndexedValue]s where the nth value holds the nth
+ * element of [iterable] and its index. See [enumerate].
+ */
+// This was inspired by MappedIterable internal to Dart collections.
+class EnumerateIterable<V> extends IterableBase<IndexedValue<V>> {
+  final Iterable<V> _iterable;
+
+  EnumerateIterable(this._iterable);
+
+  Iterator<V> get iterator => new EnumerateIterator<V>(_iterable.iterator);
+
+  // Length related functions are independent of the mapping.
+  int get length => _iterable.length;
+  bool get isEmpty => _iterable.isEmpty;
+
+  // Index based lookup can be done before transforming.
+  IndexedValue<V> get first => new IndexedValue<V>(0, _iterable.first);
+  IndexedValue<V> get last => new IndexedValue<V>(length - 1, _iterable.last);
+  IndexedValue<V> get single => new IndexedValue<V>(0, _iterable.single);
+  IndexedValue<V> elementAt(int index) =>
+      new IndexedValue<V>(index, _iterable.elementAt(index));
+}
+
+/** The [Iterator] returned by [EnumerateIterable.iterator]. */
+class EnumerateIterator<V> extends Iterator<IndexedValue<V>> {
+  final Iterator<IndexedValue<V>> _iterator;
+  int _index = 0;
+  IndexedValue<V> _current;
+
+  EnumerateIterator(this._iterator);
+
+  IndexedValue<V> get current => _current;
+
+  bool moveNext() {
+    if (_iterator.moveNext()) {
+      _current = new IndexedValue(_index++, _iterator.current);
+      return true;
+    }
+    _current = null;
+    return false;
+  }
+}