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

client/web/minimap-element.html

 <polymer-element name="dartdoc-minimap">
   <template>
     <template if="{{itemsToShow.isNotEmpty}}">
       <li>
         <a id="{{camelCaseName}}" href="{{categoryLink}}">{{category.name}}</a>
       </li>
       <ul class="minimap-sublist collapse in">
         <template repeat="{{itemsToShow}}">
-          <li><a href="#{{anchorHref}}">{{decoratedName}}</a></li>
+          <li><a href="{{prefixedAnchorHref}}">{{decoratedName}}</a></li>
         </template>
       </ul>
     </template>
   </template>
   <script type="application/dart" src="minimap-element.dart"></script>
 </polymer-element>