Adds legacyOptimizations flag

Set using `Polymer.setLegacyOptimizations(true)`. This flag enables optimizaitons including, always stripping whitespace from templates and avoiding cloning element templates (assumes no inheritance is used)

Also re-enables dir-mixin.

lib/mixins/dir-mixin.html

@@ -79,7 +79,6 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @memberof Polymer
      */
     Polymer.DirMixin = Polymer.dedupingMixin((base) => {
-      return base;
 
       if (!observer) {
         getRTL();

lib/mixins/element-mixin.html

@@ -314,8 +314,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
             let t = document.createElement('template');
             t.innerHTML = template;
             template = t;
-          // } else {
-          //   template = template.cloneNode(true);
+          } else if (!Polymer.legacyOptimizations) {
+             template = template.cloneNode(true);
           }
         }
 
@@ -409,7 +409,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * The `importPath` property is also set on element instances and can be
        * used to create bindings relative to the import path.
        *
-       * For elements defined in ES modules, users should implement 
+       * For elements defined in ES modules, users should implement
        * `static get importMeta() { return import.meta; }`, and the default
        * implementation of `importPath` will  return `import.meta.url`'s path.
        * For elements defined in HTML imports, this getter will return the path
@@ -428,7 +428,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
             this._importPath = Polymer.ResolveUrl.pathFromUrl(meta.url);
           } else {
             const module = Polymer.DomModule && Polymer.DomModule.import(/** @type {PolymerElementConstructor} */ (this).is);
-            this._importPath = (module && module.assetpath) || 
+            this._importPath = (module && module.assetpath) ||
               Object.getPrototypeOf(/** @type {PolymerElementConstructor}*/ (this).prototype).constructor.importPath;
           }
         }

lib/mixins/template-stamp.html

@@ -203,9 +203,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         if (!template._templateInfo) {
           let templateInfo = template._templateInfo = {};
           templateInfo.nodeInfoList = [];
-          templateInfo.stripWhiteSpace = true;
-            // (outerTemplateInfo && outerTemplateInfo.stripWhiteSpace) ||
-            // template.hasAttribute('strip-whitespace');
+          templateInfo.stripWhiteSpace = Polymer.legacyOptimizations ||
+            (outerTemplateInfo && outerTemplateInfo.stripWhiteSpace) ||
+            template.hasAttribute('strip-whitespace');
           this._parseTemplateContent(template, templateInfo, {parent: null});
         }
         return template._templateInfo;

lib/utils/settings.html

@@ -114,5 +114,19 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
   Polymer.setPassiveTouchGestures = function(usePassive) {
     Polymer.passiveTouchGestures = usePassive;
   };
+
+  let legacyOptimizations = false;
+  Polymer.legacyOptimizations = legacyOptimizations;
+  /**
+   * Sets `legacyOptimizations` globally for all elements. Enables
+   * optimizations when only legacy Polymer() style elements are used.
+   *
+   * @memberof Polymer
+   * @param {boolean} useLegacyOptimizations enable or disable legacy optimizations globally.
+   * @return {void}
+   */
+  Polymer.setLegacyOptimizations = function(useLegacyOptimizations) {
+    Polymer.legacyOptimizations = useLegacyOptimizations;
+  };
 })();
 </script>