Add reasoning for suppress missingProperties

lib/legacy/legacy-element-mixin.html

@@ -91,7 +91,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
        * passed to `Polymer({...})`) to the static API.
        *
        * @return {!Object} The `import.meta` object set on the prototype
-       * @suppress {missingProperties}
+       * @suppress {missingProperties} `this` is always in the instance in
+       *  closure for some reason even in a static method, rather than the class
        */
       static get importMeta() {
         return this.prototype.importMeta;