message: Prepare CLI 1.7.0-pre.6 release.
diff: <!-- ## Unreleased -->
  <!-- Add new, unreleased items here. -->
  +## v1.7.0-pre.6 [04-09-2018]
  +* Bring in latest polymer-analyzer and polymer-build fixes to node resolution
  + and babel helpers.
  +
  ## v1.7.0-pre.4 [04-05-2018]
  - `build`
  - ES modules are now be bundled.
  