Fix allowTemplateFromDomModule opt-in

lib/mixins/element-mixin.js

@@ -280,7 +280,9 @@ export const ElementMixin = dedupingMixin(base => {
    */
   function getTemplateFromDomModule(is) {
     let template = null;
-    if (is && allowTemplateFromDomModule) {
+    // Under strictTemplatePolicy in 3.x+, dom-module lookup is only allowed
+    // when opted-in via allowTemplateFromDomModule
+    if (is && (!strictTemplatePolicy || allowTemplateFromDomModule)) {
       template = DomModule.import(is, 'template');
       // Under strictTemplatePolicy, require any element with an `is`
       // specified to have a dom-module