message: Prepare CHANGELOG for 3.0.0-pre.16 release.
diff: @@ -5,12 +5,14 @@ All notable changes to this project will be documented in this file.
  The format is based on [Keep a Changelog](http://keepachangelog.com/)
  and this project adheres to [Semantic Versioning](http://semver.org/).
  -## Unreleased
  +<!-- ## Unreleased -->
  +<!-- Add new, unreleased changes here. -->
  +
  +## [3.0.0-pre.16] - 2018-03-15
  * Import URLs which are resolved, but for which the URL loader returns
  `canLoad() === false` result in a `not-loadable` Warning instead of
  `could-not-load`, which is reserved for other failures, such as parser
  errors or load errors.
  -<!-- Add new, unreleased changes here. -->
  ## [3.0.0-pre.15] - 2018-03-15
  * Now recognizes classes, behaviors, elements, and mixins that are
  