Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/lib/visitor.dart

Index: pkg/polymer_expressions/lib/visitor.dart
diff --git a/pkg/polymer_expressions/lib/visitor.dart b/pkg/polymer_expressions/lib/visitor.dart
index 8c4118df150aa432128a079f8e874285fb7829c8..7bf8053854e814224dc8d901f5788a669164693f 100644
--- a/pkg/polymer_expressions/lib/visitor.dart
+++ b/pkg/polymer_expressions/lib/visitor.dart
@@ -22,6 +22,7 @@ abstract class Visitor {
   visitUnaryOperator(UnaryOperator o);
   visitTernaryOperator(TernaryOperator o);
   visitInExpression(InExpression c);
+  visitAsExpression(AsExpression c);
 }
 
 class RecursiveVisitor extends Visitor {
@@ -123,4 +124,10 @@ class RecursiveVisitor extends Visitor {
     visit(c.right);
     visitExpression(c);
   }
+
+  visitAsExpression(AsExpression c) {
+    visit(c.left);
+    visit(c.right);
+    visitExpression(c);
+  }
 }