Bump acorn from 6.4.0 to 6.4.2

Bumps [acorn](https://github.com/acornjs/acorn) from 6.4.0 to 6.4.2.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.4.0...6.4.2)

---
updated-dependencies:
- dependency-name: acorn
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

yarn.lock

@@ -1357,9 +1357,9 @@ acorn-walk@^6.1.1:
   integrity sha1-Ejy487hMIXHx9/slJhWxx4prGow=
 
 acorn@^6.0.7, acorn@^6.2.1:
-  version "6.4.0"
-  resolved "https://registry.npm.taobao.org/acorn/download/acorn-6.4.0.tgz?cache=0&other_urls=https%3A%2F%2Fregistry.npm.taobao.org%2Facorn%2Fdownload%2Facorn-6.4.0.tgz#b659d2ffbafa24baf5db1cdbb2c94a983ecd2784"
-  integrity sha1-tlnS/7r6JLr12xzbsslKmD7NJ4Q=
+  version "6.4.2"
+  resolved "https://registry.yarnpkg.com/acorn/-/acorn-6.4.2.tgz#35866fd710528e92de10cf06016498e47e39e1e6"
+  integrity sha512-XtGIhXwF8YM8bJhGxG5kXgjkEuNGLTkoYqVE+KMR+aspr4KGYmKYg7yUe3KghyQ9yheNwLnjmzh/7+gfDBmHCQ==
 
 acorn@^7.1.0:
   version "7.1.0"