Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/test/tokenizer_test.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 tokenizer_test;
 
 import 'package:polymer_expressions/tokenizer.dart';
 import 'package:unittest/unittest.dart';
 
 main() {
@@ skipping 56 matching lines @@
           t(IDENTIFIER_TOKEN, 'c')]);
     });
 
     test('should tokenize an iterate expression with "in" keyword', () {
       expectTokens('item in items', [
           t(IDENTIFIER_TOKEN, 'item'),
           t(KEYWORD_TOKEN, 'in'),
           t(IDENTIFIER_TOKEN, 'items')]);
     });
 
+    test('should takenize an "as" expression', () {
+      expectTokens('a as b', [
+          t(IDENTIFIER_TOKEN, 'a'),
+          t(KEYWORD_TOKEN, 'as'),
+          t(IDENTIFIER_TOKEN, 'b')]);
+    });
+
     test('should tokenize keywords', () {
       expectTokens('in', [t(KEYWORD_TOKEN, 'in')]);
       expectTokens('this', [t(KEYWORD_TOKEN, 'this')]);
     });
 
     test('should tokenize groups', () {
       expectTokens('a(b)[]{}', [
           t(IDENTIFIER_TOKEN, 'a'),
           t(GROUPER_TOKEN, '('),
           t(IDENTIFIER_TOKEN, 'b'),
@@ skipping 114 matching lines @@
     } else {
       if (item.length != matchers.length) {
         mismatchDescription.add('wrong lengths');
       } else {
         mismatchDescription.add('was ').addDescriptionOf(item);
       }
     }
   }
 
 }