Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/lib/parser.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.parser;
 
 import 'tokenizer.dart';
 import 'expression.dart';
 
 const _UNARY_OPERATORS = const ['+', '-', '!'];
@@ skipping 46 matching lines @@
         } else if (_token.value == '[') {
           var indexExpr = _parseIndex();
           left = _astFactory.index(left, indexExpr);
         } else {
           break;
         }
       } else if (_token.kind == DOT_TOKEN) {
         _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);
       } else {
         break;
       }
     }
     return left;
   }
 
@@ skipping 55 matching lines @@
 
   Expression _parsePrimary() {
     var kind = _token.kind;
     switch (kind) {
       case KEYWORD_TOKEN:
         var keyword = _token.value;
         if (keyword == 'this') {
           _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:
         return _parseInvokeOrIdentifier();
       case STRING_TOKEN:
         return _parseString();
       case INTEGER_TOKEN:
         return _parseInteger();
       case DECIMAL_TOKEN:
         return _parseDecimal();
@@ skipping 41 matching lines @@
     return new MapLiteral(entries);
   }
 
   MapLiteralEntry _parseMapLiteralEntry() {
     var key = _parseString();
     _advance(COLON_TOKEN, ':');
     var value = _parseExpression();
     return _astFactory.mapLiteralEntry(key, value);
   }
 
-  InExpression _parseComprehension(Expression left) {
+  InExpression _parseInExpression(Expression left) {
     assert(_token.value == 'in');
     if (left is! Identifier) {
       throw new ParseException(
           "in... statements must start with an identifier");
     }
     _advance();
     var right = _parseExpression();
     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();
       return _astFactory.literal(true);
     }
     if (_token.value == 'false') {
       _advance();
       return _astFactory.literal(false);
     }
     if (_token.value == 'null') {
@@ skipping 64 matching lines @@
     return value;
   }
 
   Literal<double> _parseDecimal([String prefix = '']) {
     var value = _astFactory.literal(double.parse('$prefix${_token.value}'));
     _advance();
     return value;
   }
 
 }