Update Acorn

Author: kristoferbaxter

package.json

@@ -32,9 +32,9 @@
     "rollup": ">= 0.67.4"
   },
   "dependencies": {
-    "acorn": "6.3.0",
+    "acorn": "7.1.0",
     "acorn-dynamic-import": "4.0.0",
-    "acorn-walk": "6.2.0",
+    "acorn-walk": "7.0.0",
     "google-closure-compiler": "20191111.0.0",
     "magic-string": "0.25.4",
     "temp-write": "4.0.0"