Refactor of PolymerExpressions. Adds "as" expressions.

pkg/template_binding/lib/src/mustache_tokens.dart

Index: pkg/template_binding/lib/src/mustache_tokens.dart
diff --git a/pkg/template_binding/lib/src/mustache_tokens.dart b/pkg/template_binding/lib/src/mustache_tokens.dart
index be84d26205cba07597410dc02446e1604e4f646d..d7bb83423ed70b8a8c8ee83e0cfb828fe97ae89f 100644
--- a/pkg/template_binding/lib/src/mustache_tokens.dart
+++ b/pkg/template_binding/lib/src/mustache_tokens.dart
@@ -112,9 +112,14 @@ class MustacheTokens {
       var pathString = s.substring(startIndex + 2, endIndex).trim();
       tokens.add(oneTime); // ONETIME?
       onlyOneTime = onlyOneTime && oneTime;
-      tokens.add(new PropertyPath(pathString)); // PATH
       var delegateFn = delegate == null ? null :
           delegate.prepareBinding(pathString, name, node);
+      // Don't try to parse the expression if there's a prepareBinding function
+      if (delegateFn == null) {
+        tokens.add(new PropertyPath(pathString)); // PATH
+      } else {
+        tokens.add(null); // PATH
+      }
       tokens.add(delegateFn);
 
       lastIndex = endIndex + 2;