feat(benchmarks): add polymer js 0.8-preview benchmark

Limitations because of preview status (see #960):

- does not yet use ShadowDOM
- does not use a builtin conditional like `if`
- uses a temporary bower repository

Closes #943

modules/benchmarks_external/src/index.html

@@ -8,6 +8,9 @@
     <li>
       <a href="tree/tree_benchmark.html">Tree benchmark</a>
     </li>
+    <li>
+      <a href="tree/polymer/index.html">Polymer Tree benchmark</a>
+    </li>
     <li>
       <a href="largetable/largetable_benchmark.html">Largetable benchmark</a>
     </li>