fix logic for detecting when in an auto-binding element

pkg/polymer/lib/src/build/linter.dart

Index: pkg/polymer/lib/src/build/linter.dart
diff --git a/pkg/polymer/lib/src/build/linter.dart b/pkg/polymer/lib/src/build/linter.dart
index 39cea9b71101b48793787589c62e7a6fe52a2e18..303b020286228c6d8356f8edcaa34c54e9fe4e2d 100644
--- a/pkg/polymer/lib/src/build/linter.dart
+++ b/pkg/polymer/lib/src/build/linter.dart
@@ -195,12 +195,13 @@ class _LinterVisitor extends TreeVisitor {
       case 'script': _validateScriptElement(node); break;
       case 'template':
         var isTag = node.attributes['is'];
+        var oldInAutoBindingElement = _inAutoBindingElement;
         if (isTag != null && AUTO_BINDING_ELEMENTS.contains(isTag)) {
           _inAutoBindingElement = true;
         }
         _validateNormalElement(node);
         super.visitElement(node);
-        _inAutoBindingElement = false;
+        _inAutoBindingElement = oldInAutoBindingElement;
         break;
       default:
         _validateNormalElement(node);