don't trap id for marshalling if it's a binding directive + test

test/unit/bind-elements.html

@@ -1,5 +1,5 @@
 <template>
-  <div id="boundChild" 
+  <div id="boundChild"
     value="{{value}}"
     negvalue="{{!bool}}"
     attrvalue$="{{attrvalue}}"
@@ -15,6 +15,7 @@
     custom-event-object-value="{{customEventObject.value::change}}">
     Test
     </div>
+    <span id="{{boundId}}"></span>
 </template>
 <script>
   Polymer({
@@ -72,6 +73,10 @@
       customEventObject: {
         type: Object,
         value: function() { return {}; }
+      },
+      boundId: {
+        type: String,
+        value: 'span'
       }
     },
     observers: [
@@ -182,7 +187,7 @@
 </script>
 
 <template>
-  <x-basic id="basic1" 
+  <x-basic id="basic1"
     value="{{boundvalue}}"
     notifyingvalue="{{boundnotifyingvalue}}"
     camel-notifying-value="{{boundnotifyingvalue}}"