message: Prepare 3.0.0-pre.21 release.
diff: @@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
  <!-- ## Unreleased -->
  <!-- Add new, unreleased changes here. -->
  +## [3.0.0-pre.21] - 2018-04-05
  +* Clarify module resolution error message.
  +
  ## [3.0.0-pre.20] - 2018-04-03
  * Added support for parsing `import.meta` in JavaScript modules.
  