Avoid polymer partial initialization of bindings

runtime/observatory/lib/src/elements/field_view.html

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="instance_ref.html">
 
 <polymer-element name="field-view">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <nav-bar>
       <top-nav-menu></top-nav-menu>
       <vm-nav-menu vm="{{ field.isolate.vm }}"></vm-nav-menu>
       <isolate-nav-menu isolate="{{ field.isolate }}"></isolate-nav-menu>
@@ skipping 42 matching lines @@
           <div class="memberItem"
                title="The types observed for this field at runtime.  Fields that are observed to have a single type at runtime or to never be null may allow for additional optimization.">
             <div class="memberName">observed types</div>
             <div class="memberValue">
               <template if="{{ field.guardClass == 'dynamic' }}">
                 various
               </template>
               <template if="{{ field.guardClass == 'unknown' }}">
                 none
               </template>
-              <template if="{{ field.guardClass != 'unknown' &&
+              <template if="{{ field.guardClass != null &&
+                            field.guardClass != 'unknown' &&
                             field.guardClass != 'dynamic' }}">
                 <class-ref ref="{{ field.guardClass }}"></class-ref>
                 <template if="{{ field.guardNullable }}">
                   &mdash; null observed
                 </template>
                 <template if="{{ !field.guardNullable }}">
                   &mdash; null not observed
                 </template>
               </template>
             </div>
@@ skipping 16 matching lines @@
                     startPos="{{ field.location.tokenPos }}"
                     endPos="{{ field.location.tokenPos }}">
       </script-inset>
     </div>
 
     <view-footer></view-footer>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="field_view.dart"></script>