Adds syncInitialRender setting

This separates the behavior from `legacyOptimizations`. Performs synchronous initial rendering under ShadyDOM when this flag is set.

lib/mixins/element-mixin.js

@@ -9,7 +9,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 */
 import '../utils/boot.js';
 
-import { rootPath, strictTemplatePolicy, allowTemplateFromDomModule, legacyOptimizations } from '../utils/settings.js';
+import { rootPath, strictTemplatePolicy, allowTemplateFromDomModule, legacyOptimizations, syncInitialRender } from '../utils/settings.js';
 import { dedupingMixin } from '../utils/mixin.js';
 import { stylesFromTemplate, stylesFromModuleImports } from '../utils/style-gather.js';
 import { pathFromUrl, resolveCss, resolveUrl } from '../utils/resolve-url.js';
@@ -659,7 +659,7 @@ export const ElementMixin = dedupingMixin(base => {
             n.attachShadow({mode: 'open'});
           }
           n.shadowRoot.appendChild(dom);
-          if (legacyOptimizations && window.ShadyDOM) {
+          if (syncInitialRender && window.ShadyDOM) {
             ShadyDOM.flushInitial(n.shadowRoot);
           }
           return n.shadowRoot;

lib/utils/settings.js

@@ -140,4 +140,22 @@ export let legacyOptimizations = false;
  */
 export const setLegacyOptimizations = function(useLegacyOptimizations) {
   legacyOptimizations = useLegacyOptimizations;
+};
+
+/**
+ * Setting to perform initial rendering synchronously when running under ShadyDOM.
+ * This matches the behavior of Polymer 1.
+ */
+export let syncInitialRender = false;
+
+/**
+ * Sets `syncInitialRender` globally for all elements to enable synchronous
+ * initial rendering.
+ *
+ * @param {boolean} useSyncInitialRender enable or disable synchronous initial
+ * rendering globally.
+ * @return {void}
+ */
+export const setSyncInitialRender = function(useSyncInitialRender) {
+  syncInitialRender = useSyncInitialRender;
 };