Log Message:
npm: updated to 6.9.0

6.9.0:
FEATURES
* Time traveling installs using the --before flag.
* Add support for package aliases. This allows packages to be installed under a
  different directory than the package name listed in package.json, and adds a
  new dependency type to allow this to be done for registry dependencies.
* Always save package-lock.json when using --package-lock-only.
* Make empty-string run-scripts run successfully as a no-op.
* Match git semver ranges when flattening the tree.
* Re-enable updating local packages.

BUGFIXES
* Set modified to undefined in npm view when time is not available. This
  fixes a bug where npm view would crash on certain third-party registries.
* Print out tar version in install.sh only when the flag is supported not all
  the tar implementations support --version flag. This allows the install script
  to work in OpenBSD, for example.
* Fix typo in error message for npm stars.
* Strip version info from pkg on E404. This improves the error messaging format.

DOCS
* Add npm add as alias to npm install in docs.
* Fix link to RFC 10 in the changelog.
* Describe exit codes in npm-audit docs.