Implement opt-in strictTemplatePolicy (flag TBD)

- disable dom-bind
- disable dom-module re-registration
- require elements with no template to implement null template
- disable templatizer of templates not stamped in trusted polymer template

lib/elements/dom-bind.html

@@ -56,6 +56,9 @@
 
       constructor() {
         super();
+        if (window.strictTemplatePolicy) {
+          throw new Error(`strictTemplatePolicy: dom-bind not allowed`);
+        }
         this.root = null;
         this.$ = null;
         this.__children = null;

lib/elements/dom-module.html

@@ -124,6 +124,10 @@
     register(id) {
       id = id || this.id;
       if (id) {
+        if (window.strictTemplatePolicy && findModule(id)) {
+          modules[id] = lcModules[id.toLowerCase()] = null;
+          throw new Error(`strictTemplatePolicy: dom-module ${id} registered twice`);
+        }
         this.id = id;
         // store id separate from lowercased id so that
         // in all cases mixedCase id will stored distinctly

lib/legacy/class.html

@@ -157,7 +157,10 @@
          */
         static get template() {
           // get template first from any imperative set in `info._template`
-          return info._template ||
+          if (info._template !== undefined) {
+            return info._template;
+          }
+          const template = info._template ||
             // next look in dom-module associated with this element's is.
             Polymer.DomModule && Polymer.DomModule.import(this.is, 'template') ||
             // next look for superclass template (note: use superclass symbol
@@ -166,6 +169,10 @@
             // finally fall back to `_template` in element's prototype.
             this.prototype._template ||
             null;
+          if (window.strictTemplatePolicy && !template) {
+            throw new Error(`strictTemplatePolicy: expecting dom-module or null _template for ${this.is}`);
+          }
+          return template;
         }
 
         /**

lib/mixins/element-mixin.html

@@ -386,12 +386,16 @@
        */
       static get template() {
         if (!this.hasOwnProperty(JSCompiler_renameProperty('_template', this))) {
-          this._template = Polymer.DomModule && Polymer.DomModule.import(
+          const template = Polymer.DomModule && Polymer.DomModule.import(
             /** @type {PolymerElementConstructor}*/ (this).is, 'template') ||
             // note: implemented so a subclass can retrieve the super
             // template; call the super impl this way so that `this` points
             // to the superclass.
             Object.getPrototypeOf(/** @type {PolymerElementConstructor}*/ (this).prototype).constructor.template;
+          if (window.strictTemplatePolicy && this.is && !template) {
+            throw new Error(`strictTemplatePolicy: expecting dom-module or null template for ${this.is}`);
+          }
+          this._template = template;
         }
         return this._template;
       }

lib/utils/templatize.html

@@ -500,6 +500,9 @@
        * @suppress {invalidCasts}
        */
       templatize(template, owner, options) {
+        if (window.strictTemplatePolicy && !owner._methodHost) {
+          throw new Error('strictTemplatePolicy: template owner not trusted');
+        }
         options = /** @type {!TemplatizeOptions} */(options || {});
         if (template.__templatizeOwner) {
           throw new Error('A <template> can only be templatized once');