fix dart2js warnings in polymer_expressions pkg

pkg/polymer_expressions/lib/filter.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.
 
 library polymer_expressions.filter;
 
 typedef Object Filter(Object value);
 
 abstract class Transformer<T, V> {
 
   T forward(V v);
   V reverse(T t);
-  Transformer<V, T> get inverse => new _InverseTransformer(this);
+  Transformer<V, T> get inverse => new _InverseTransformer<V, T>(this);
 }
 
-class _InverseTransformer<T, V> implements Transformer<T, V> {
-  final Transformer<V, T> _t;
+class _InverseTransformer<V, T> implements Transformer<V, T> {

[Jennifer Messerly, 2013/10/14 19:48:44]

note: this isn't intended to change anything, just make it more obvious what's
going on by keeping the meaning of T and V constant with respect to the original
Transformer.

(I thought it might help analyzer, but it doesn't)

+  final Transformer<T, V> _t;
   _InverseTransformer(this._t);
 
-  T forward(V v) => _t.reverse(v);
-  V reverse(T t) => _t.forward(t);
-  Transformer<V, T> get inverse => _t;
+  V forward(T v) => _t.reverse(v);
+  T reverse(V t) => _t.forward(t);
+  Transformer<T, V> get inverse => _t;
 }