Silently fail when assigning to properties on null

pkg/polymer_expressions/lib/eval.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library polymer_expressions.eval;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:observe/observe.dart';
@@ skipping 116 matching lines @@
   // transform the values backwards through the filters
   for (var filterExpr in filters) {
     var filter = eval(filterExpr, scope);
     if (filter is! Transformer) {
       throw new EvalException("filter must implement Transformer: $filterExpr");
     }
     value = filter.reverse(value);
   }
   // make the assignment
   var o = eval(expression, scope);
-  if (o == null) throw new EvalException("Can't assign to null: $expression");
+
+  // can't assign to a property on a null LHS object. Silently fail.
+  if (o == null) return null;
+
   if (isIndex) {
     o[property] = value;
   } else {
     smoke.write(o, smoke.nameToSymbol(property), value);
   }
   return value;
 }
 
 
 /**
@@ skipping 566 matching lines @@
   accept(Visitor v) => v.visitInvoke(this);
 }
 
 _toBool(v) => (v == null) ? false : v;
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }