fix dart2js warnings in polymer_expressions pkg

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);
+  Iterator<IndexedValue<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;
+  final Iterator<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;
   }
 }