Refactor of PolymerExpressions. Adds "as" 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 {
   visit(Expression s) => s.accept(this);
   visitEmptyExpression(EmptyExpression e);
   visitParenthesizedExpression(ParenthesizedExpression e);
   visitGetter(Getter i);
   visitIndex(Index i);
   visitInvoke(Invoke i);
   visitLiteral(Literal l);
   visitListLiteral(ListLiteral l);
   visitMapLiteral(MapLiteral l);
   visitMapLiteralEntry(MapLiteralEntry l);
   visitIdentifier(Identifier i);
   visitBinaryOperator(BinaryOperator o);
   visitUnaryOperator(UnaryOperator o);
   visitTernaryOperator(TernaryOperator o);
   visitInExpression(InExpression c);
+  visitAsExpression(AsExpression c);
 }
 
 abstract class RecursiveVisitor extends Visitor {
   visitExpression(Expression e);
 
   visitEmptyExpression(EmptyExpression e) => visitExpression(e);
 
   visitParenthesizedExpression(ParenthesizedExpression e) {
     visit(e.child);
     visitExpression(e);
@@ skipping 60 matching lines @@
     visit(o.trueExpr);
     visit(o.falseExpr);
     visitExpression(o);
   }
 
   visitInExpression(InExpression c) {
     visit(c.left);
     visit(c.right);
     visitExpression(c);
   }
+
+  visitAsExpression(AsExpression c) {
+    visit(c.left);
+    visit(c.right);
+    visitExpression(c);
+  }
 }