Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/lib/expression.dart

Index: pkg/polymer_expressions/lib/expression.dart
diff --git a/pkg/polymer_expressions/lib/expression.dart b/pkg/polymer_expressions/lib/expression.dart
index 54adfd7579bc7c2a7730309996b63786f1de26b2..ec9e950ff80dafa46e158caac741732fa7c5901f 100644
--- a/pkg/polymer_expressions/lib/expression.dart
+++ b/pkg/polymer_expressions/lib/expression.dart
@@ -24,6 +24,7 @@ Index index(Expression e, Expression a) => new Index(e, a);
 Invoke invoke(Expression e, String m, List<Expression> a) =>
     new Invoke(e, m, a);
 InExpression inExpr(Expression l, Expression r) => new InExpression(l, r);
+AsExpression asExpr(Expression l, Expression r) => new AsExpression(l, r);
 TernaryOperator ternary(Expression c, Expression t, Expression f) =>
     new TernaryOperator(c, t, f);
 
@@ -59,6 +60,8 @@ class AstFactory {
       new Invoke(e, m, a);
 
   InExpression inExpr(Expression l, Expression r) => new InExpression(l, r);
+
+  AsExpression asExpr(Expression l, Expression r) => new AsExpression(l, r);
 }
 
 /// Base class for all expressions
@@ -228,6 +231,22 @@ class InExpression extends Expression {
   int get hashCode => _JenkinsSmiHash.hash2(left.hashCode, right.hashCode);
 }
 
+class AsExpression extends Expression {
+  final Expression left;
+  final Expression right;
+
+  AsExpression(this.left, this.right);
+
+  accept(Visitor v) => v.visitAsExpression(this);
+
+  String toString() => '($left as $right)';
+
+  bool operator ==(o) => o is AsExpression && o.left == left
+      && o.right == right;
+
+  int get hashCode => _JenkinsSmiHash.hash2(left.hashCode, right.hashCode);
+}
+
 class Index extends Expression {
   final Expression receiver;
   final Expression argument;