Correctly generate code when using transformers in polymer-expressions.

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

Index: pkg/polymer/lib/src/build/script_compactor.dart
diff --git a/pkg/polymer/lib/src/build/script_compactor.dart b/pkg/polymer/lib/src/build/script_compactor.dart
index 6a9fc420eb6e9258a69428a3f90f5960068a2f4d..40546489798e9fed789df9b58a58477c70a77e2e 100644
--- a/pkg/polymer/lib/src/build/script_compactor.dart
+++ b/pkg/polymer/lib/src/build/script_compactor.dart
@@ -529,8 +529,11 @@ class _SubExpressionVisitor extends pe.RecursiveVisitor {
 
   void preVisitExpression(e) {
     // For two-way bindings the outermost expression may be updated, so we need
-    // both the getter and the setter, but subexpressions only need the getter.
-    // So we exclude setters as soon as we go deeper in the tree.
+    // both the getter and the setter, but we only need the getter for
+    // subexpressions. We exclude setters as soon as we go deeper in the tree,
+    // except when we see a filter (that can potentially be a two-way
+    // transformer).
+    if (e is pe.BinaryOperator && e.operator == '|') return;
     _includeSetter = false;
   }