Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/lib/tokenizer.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.tokenizer;
 
 const int _TAB = 9;
 const int _LF = 10;
 const int _VTAB = 11;
 const int _FF = 12;
@@ skipping 41 matching lines @@
 
 const _OPERATORS = const [_PLUS, _MINUS, _STAR, _SLASH, _BANG, _AMPERSAND,
                           _PERCENT, _LT, _EQ, _GT, _QUESTION, _CARET, _BAR];
 
 const _GROUPERS = const [_OPEN_PAREN, _CLOSE_PAREN,
                          _OPEN_SQUARE_BRACKET, _CLOSE_SQUARE_BRACKET,
                          _OPEN_CURLY_BRACKET, _CLOSE_CURLY_BRACKET];
 
 const _TWO_CHAR_OPS = const ['==', '!=', '<=', '>=', '||', '&&'];
 
-const _KEYWORDS = const ['in', 'this'];
+const _KEYWORDS = const ['as', 'in', 'this'];
 
 const _PRECEDENCE = const {
   '!':  0,
   ':':  0,
   ',':  0,
   ')':  0,
   ']':  0,
   '}':  0, // ?
   '?':  1,
   '||': 2,
@@ skipping 219 matching lines @@
     _tokens.add(new Token(GROUPER_TOKEN, value, _PRECEDENCE[value]));
     _advance();
   }
 }
 
 class ParseException implements Exception {
   final String message;
   ParseException(this.message);
   String toString() => "ParseException: $message";
 }