Refactor of PolymerExpressions. Adds "as" expressions.

pkg/polymer_expressions/test/tokenizer_test.dart

Index: pkg/polymer_expressions/test/tokenizer_test.dart
diff --git a/pkg/polymer_expressions/test/tokenizer_test.dart b/pkg/polymer_expressions/test/tokenizer_test.dart
index a93dce73ba615fdc61edaaad20d7c7e761bf5da8..05cfffec2f86fd4569c4e455f201fc3c438b6f9b 100644
--- a/pkg/polymer_expressions/test/tokenizer_test.dart
+++ b/pkg/polymer_expressions/test/tokenizer_test.dart
@@ -74,6 +74,13 @@ main() {
           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')]);