Fix strong mode warnings.

lib/src/iterable_zip.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.
 
 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)
+class IterableZip<T> extends IterableBase<List<T>> {
+  final Iterable<Iterable<T>> _iterables;
+
+  IterableZip(Iterable<Iterable<T>> 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);
+  Iterator<List<T>> get iterator {
+    var iterators = _iterables.map((x) => x.iterator).toList(growable: false);
     // TODO(lrn): Return an empty iterator directly if iterators is empty?
-    return new _IteratorZip(iterators);
+    return new _IteratorZip<T>(iterators);
   }
 }
 
-class _IteratorZip implements Iterator<List> {
-  final List<Iterator> _iterators;
-  List _current;
-  _IteratorZip(List iterators) : _iterators = iterators;
+class _IteratorZip<T> implements Iterator<List<T>> {
+  final List<Iterator<T>> _iterators;
+  List<T> _current;
+
+  _IteratorZip(List<Iterator<T>> 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;
+  List<T> get current => _current;
 }