Add property doc for Polymer.rootPath, move to element-mixim

lib/mixins/element-mixin.html

@@ -788,5 +788,18 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
   };
 
+  /**
+   * Globally settable property that is automatically assigned to
+   * `Polymer.ElementMixin` instances, useful for binding in templates to
+   * make URL's relative to an application's root.  Defaults to the main
+   * document URL, but can be overridden by users.  It may be useful to set
+   * `Polymer.rootPath` to provide a stable application mount path when
+   * using client side routing.
+   *
+   * @memberof Polymer
+   */
+  Polymer.rootPath = Polymer.rootPath ||
+    Polymer.ResolveUrl.pathFromUrl(document.baseURI || window.location.href);
+
 })();
 </script>

lib/utils/resolve-url.html

@@ -102,10 +102,6 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       pathFromUrl: pathFromUrl
     };
 
-    // NOTE: baseURI is not supported on IE
-    Polymer.rootPath = Polymer.rootPath ||
-      pathFromUrl(document.baseURI || window.location.href);
-
   })();
 
 </script>