refactor: es6 getter/setter

packages/mixins/property.js

@@ -52,7 +52,7 @@ export function PropertyMixin(key, meta = {}) {
       metaDefault = JSON.parse(JSON.stringify(meta.default));
     }
 
-    class Property extends Superclass {
+    return class Property extends Superclass {
 
       constructor(...args) {
         super(...args);
@@ -61,22 +61,18 @@ export function PropertyMixin(key, meta = {}) {
         }
       }
 
-    }
-
-    Object.defineProperty(Property.prototype, key, {
-      enumerable: true,
-      get: function() {
+      get [key]() {
         return meta.get.call(this);
-      },
-      set: function (value) {
+      }
+
+      set [key](value) {
         const old = meta.get.call(this);
         meta.set.call(this, value);
         if (meta.track && meta.emit && !meta.eq.call(this, old, value)) {
           meta.emit.call(this, `${key}Changed`, old, value);
         }
-      },
-    });
+      }
 
-    return Property;
+    }
   }
 }