Update acorn.

package-lock.json

@@ -3776,8 +3776,7 @@
       "dependencies": {
         "acorn": {
           "version": "5.7.3",
-          "resolved": "https://registry.npmjs.org/acorn/-/acorn-5.7.3.tgz",
-          "integrity": "sha512-T/zvzYRfbVojPWahDsE5evJdHb3oJoQfFbsrKM7w5Zcs++Tr257tia3BmMP8XYVjp1S9RZXQMh7gao96BlqZOw==",
+          "resolved": "",
           "dev": true
         },
         "acorn-jsx": {