PolymerExpressions: fix for null iterators in comprehensions

pkg/polymer_expressions/lib/eval.dart

Index: pkg/polymer_expressions/lib/eval.dart
diff --git a/pkg/polymer_expressions/lib/eval.dart b/pkg/polymer_expressions/lib/eval.dart
index 7341e05515025a269384602368444c5b494b5ca5..b76b3b878c935b2da386a673d299e1c3509e58da 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -595,12 +595,16 @@ call(dynamic receiver, List args) {
 }
 
 /**
- * A comprehension declaration ("a in b").
+ * A comprehension declaration ("a in b"). [identifier] is the loop variable
+ * that's added to the scope during iteration. [iterable] is the set of
+ * objects to iterate over.
  */
 class Comprehension {
   final String identifier;
   final Iterable iterable;
-  Comprehension(this.identifier, this.iterable);
+
+  Comprehension(this.identifier, Iterable iterable)
+      : iterable = (iterable != null) ? iterable : const [];
 }
 
 /**