Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/lib/parser.dart

Index: pkg/polymer_expressions/lib/parser.dart
diff --git a/pkg/polymer_expressions/lib/parser.dart b/pkg/polymer_expressions/lib/parser.dart
index 411c7ae0be5c5827bdc0b566ef37bf2e1830f8f8..4d0de87b96f0047e60b935e99e48b93b7c24303b 100644
--- a/pkg/polymer_expressions/lib/parser.dart
+++ b/pkg/polymer_expressions/lib/parser.dart
@@ -64,8 +64,14 @@ class Parser {
         _advance();
         var right = _parseUnary();
         left = _makeInvokeOrGetter(left, right);
-      } else if (_token.kind == KEYWORD_TOKEN && _token.value == 'in') {
-        left = _parseComprehension(left);
+      } else if (_token.kind == KEYWORD_TOKEN) {
+        if (_token.value == 'in') {
+          left = _parseInExpression(left);
+        } else if (_token.value == 'as') {
+          left = _parseAsExpression(left);
+        } else {
+          break;
+        }
       } else if (_token.kind == OPERATOR_TOKEN
           && _token.precedence >= precedence) {
         left = _token.value == '?' ? _parseTernary(left) : _parseBinary(left);
@@ -141,8 +147,8 @@ class Parser {
           _advance();
           // TODO(justin): return keyword node
           return _astFactory.identifier('this');
-        } else if (keyword == 'in') {
-          return null;
+        } else if (KEYWORDS.contains(keyword)) {
+          throw new ParseException('invalid keyword: $keyword');
         }
         throw new ArgumentError('unrecognized keyword: $keyword');
       case IDENTIFIER_TOKEN:
@@ -204,7 +210,7 @@ class Parser {
     return _astFactory.mapLiteralEntry(key, value);
   }
 
-  InExpression _parseComprehension(Expression left) {
+  InExpression _parseInExpression(Expression left) {
     assert(_token.value == 'in');
     if (left is! Identifier) {
       throw new ParseException(
@@ -215,6 +221,17 @@ class Parser {
     return _astFactory.inExpr(left, right);
   }
 
+  AsExpression _parseAsExpression(Expression left) {
+    assert(_token.value == 'as');
+    _advance();
+    var right = _parseExpression();
+    if (right is! Identifier) {
+      throw new ParseException(
+          "as... statements must end with an identifier");
+    }
+    return _astFactory.asExpr(left, right);
+  }
+
   Expression _parseInvokeOrIdentifier() {
     if (_token.value == 'true') {
       _advance();