fix #1066: private fields inside destructuring

internal/runtime/runtime.go

@@ -182,6 +182,9 @@ func code(isES6 bool) string {
 			setter ? setter.call(obj, value) : member.set(obj, value)
 			return value
 		}
+		export var __privateAssign = (obj, member, setter) => {
+			return { set _(value) { __privateSet(obj, member, value, setter) } }
+		}
 		export var __privateMethod = (obj, member, method) => {
 			__accessCheck(obj, member, 'access private method')
 			return method