Move undeclared property warning to element-mixin.

lib/mixins/element-mixin.js

@@ -727,7 +727,7 @@ export const ElementMixin = dedupingMixin(base => {
     }
 
     /**
-     * Overrides `PropertyAccessors` to add map of dynamic functions on
+     * Overrides `PropertyEffects` to add map of dynamic functions on
      * template info, for consumption by `PropertyEffects` template binding
      * code. This map determines which method templates should have accessors
      * created for them.
@@ -743,6 +743,32 @@ export const ElementMixin = dedupingMixin(base => {
       return super._parseTemplateContent(template, templateInfo, nodeInfo);
     }
 
+    /**
+     * Overrides `PropertyEffects` to warn on use of undeclared properties in
+     * template.
+     *
+     * @param {Object} templateInfo Template metadata to add effect to
+     * @param {string} prop Property that should trigger the effect
+     * @param {Object=} effect Effect metadata object
+     * @return {void}
+     * @protected
+     */
+    static _addTemplatePropertyEffect(templateInfo, prop, effect) {
+      // Warn if properties are used in template without being declared.
+      // Properties must be listed in `properties` to be included in
+      // `observedAttributes` since CE V1 reads that at registration time, and
+      // since we want to keep template parsing lazy, we can't automatically
+      // add undeclared properties used in templates to `observedAttributes`.
+      // The warning is only enabled in `legacyOptimizations` mode, since
+      // we don't want to spam existing users who might have adopted the
+      // shorthand when attribute deserialization is not important.
+      if (legacyOptimizations && !(prop in this._properties)) {
+        console.warn(`Property '${prop}' used in template but not declared in 'properties'; ` +
+          `attribute will not be observed.`);
+      }
+      return super._addTemplatePropertyEffect(templateInfo, prop, effect);
+    }
+
   }
 
   return PolymerElement;

lib/mixins/property-effects.js

@@ -2390,15 +2390,6 @@ export const PropertyEffects = dedupingMixin(superClass => {
      * @protected
      */
     static _addTemplatePropertyEffect(templateInfo, prop, effect) {
-      // `dynamicFns` is the flattened property list, so we can use that to
-      // detect non-declared properties.  Properties must be listed in
-      // `properties` to be included in `observedAttributes` since CE V1
-      // reads that at registration time, and we want to keep template parsing
-      // lazy
-      if (legacyOptimizations && !(prop in templateInfo.dynamicFns)) {
-        console.warn(`Property '${prop}' used in template but not declared in 'properties'; ` +
-          `attribute will not be observed.`);
-      }
       let hostProps = templateInfo.hostProps = templateInfo.hostProps || {};
       hostProps[prop] = true;
       let effects = templateInfo.propertyEffects = templateInfo.propertyEffects || {};