fix dart2js warnings when compiling polymer_expressions

pkg/polymer_expressions/lib/visitor.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.visitor;
 
 import 'expression.dart';
 
-abstract class Visitor<E extends Expression> {
-  visit(E s) => s.accept(this);
+abstract class Visitor {
+  visit(Expression s) => s.accept(this);
   visitEmptyExpression(EmptyExpression e);
   visitParenthesizedExpression(ParenthesizedExpression e);
   visitInvoke(Invoke i);
   visitLiteral(Literal l);
   visitMapLiteral(MapLiteral l);
   visitMapLiteralEntry(MapLiteralEntry l);
   visitIdentifier(Identifier i);
   visitBinaryOperator(BinaryOperator o);
   visitUnaryOperator(UnaryOperator o);
   visitInExpression(InExpression c);
 }
 
-abstract class RecursiveVisitor<E> extends Visitor<E> {
-  visitExpression(E e);
+abstract class RecursiveVisitor extends Visitor {
+  visitExpression(Expression e);
 
   visitEmptyExpression(EmptyExpression e) => visitExpression(e);
 
   visitParenthesizedExpression(ParenthesizedExpression e) {
     visit(e);
     visitExpression(e);
   }
 
   visitInvoke(Invoke i) {
     visit(i.receiver);
@@ skipping 32 matching lines @@
     visit(o.child);
     visitExpression(o);
   }
 
   visitInExpression(InExpression c) {
     visit(c.left);
     visit(c.right);
     visitExpression(c);
   }
 }