Factor out the anchor prefix to easily allow switching to #!

client/web/breadcrumbs.html

 <polymer-element name="dartdoc-breadcrumbs">
   <template>
     <ul class="nav navbar-nav">
       <template repeat="{{breadcrumbs}}">
-        <li><a class="btn-link" href="#{{linkHref}}">{{decoratedName}}</a></li>
+        <li><a class="btn-link" href="{{prefixedLinkHref}}">
+          {{decoratedName}}</a></li>
       </template>
       <template if="{{lastCrumb}}">
         <li class="active">
           <a class="btn-link">{{lastCrumb.decoratedName}}</a>
         </li>
       </template>
     </ul>
   </template>
   <script type="application/dart" src="breadcrumbs.dart"></script>
 </polymer-element>