fix dart2js warnings when compiling polymer_expressions

pkg/polymer_expressions/lib/eval.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.eval;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 
@@ skipping 241 matching lines @@
     _updateSelf(scope);
 
     if (!identical(_value, _oldValue)) {
       _controller.add(_value);
     }
   }
 
   String toString() => _expr.toString();
 }
 
-class Updater extends RecursiveVisitor<ExpressionObserver> {
+class Updater extends RecursiveVisitor {
   final Scope scope;
 
   Updater(this.scope);
 
   visitExpression(ExpressionObserver e) {
     e._observe(scope);
   }
 
   visitInExpression(InObserver c) {
     visit(c.right);
@@ skipping 337 matching lines @@
   Method(this.mirror, this.symbol);
 
   dynamic call(List args) => mirror.invoke(symbol, args, null).reflectee;
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }