Add lib/utils/settings.html to hold legacy settings and rootPath

lib/legacy/polymer.dom.html

@@ -8,6 +8,7 @@
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 -->
 <link rel="import" href="../utils/boot.html">
+<link rel="import" href="../utils/settings.html">
 <link rel="import" href="../utils/flattened-nodes-observer.html">
 <link rel="import" href="../utils/flush.html">
 <script>
@@ -322,15 +323,5 @@
    * @param {Polymer.Debouncer} debouncer Debouncer to enqueue
    */
   Polymer.dom.addDebouncer = Polymer.enqueueDebouncer;
-
-  // expose BC settings.
-  let settings = Polymer.Settings || {};
-  settings.useShadow = !(window.ShadyDOM);
-  settings.useNativeCSSProperties =
-    Boolean(!window.ShadyCSS || window.ShadyCSS.nativeCss);
-  settings.useNativeCustomElements =
-    !(window.customElements.polyfillWrapFlushCallback);
-  Polymer.Settings = settings;
-
 })();
 </script>

lib/mixins/element-mixin.html

@@ -9,6 +9,7 @@
 -->
 
 <link rel="import" href="../utils/boot.html">
+<link rel="import" href="../utils/settings.html">
 <link rel="import" href="../utils/mixin.html">
 <link rel="import" href="../utils/case-map.html">
 <link rel="import" href="../utils/style-gather.html">
@@ -848,18 +849,5 @@
     }
   };
 
-  /**
-   * 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/settings.html

@@ -0,0 +1,64 @@
+<!--
+@license
+Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
+This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+Code distributed by Google as part of the polymer project is also
+subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+-->
+
+<link rel="import" href="../utils/boot.html">
+<link rel="import" href="../utils/resolve-url.html">
+
+<script>
+(function() {
+  'use strict';
+
+  /**
+   * Legacy settings.
+   * @namespace
+   * @memberof Polymer
+   */
+  const settings = Polymer.Settings || {};
+  settings.useShadow = !(window.ShadyDOM);
+  settings.useNativeCSSProperties =
+    Boolean(!window.ShadyCSS || window.ShadyCSS.nativeCss);
+  settings.useNativeCustomElements =
+    !(window.customElements.polyfillWrapFlushCallback);
+
+  /**
+   * Sets the global, legacy settings.
+   *
+   * @deprecated
+   * @memberof Polymer
+   */
+  Polymer.Settings = settings;
+
+  /**
+   * 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
+   */
+  let rootPath = Polymer.rootPath ||
+    Polymer.ResolveUrl.pathFromUrl(document.baseURI || window.location.href);
+
+  Polymer.rootPath = rootPath;
+
+  /**
+   * Sets the global rootPath property used by `Polymer.ElementMixin` and
+   * available via `Polymer.rootPath`.
+   * 
+   * @memberof Polymer
+   * @param {string} path The new root path
+   */
+  Polymer.setRootPath = function(path) {
+    Polymer.rootPath = path;
+  }
+})();
+</script>