fix(utils): espree parser isn't working with flat config

This change addresses an issue with using the espree parser with flat config.  `keysFromParser` is responsible for finding the `visitorKeys` from a combination of the parser instance, parserPath, and the results of a call to `parseForESLint`.  When using flat config, the `parserPath` will not be on the context object., and there are no results from `parseForESLint`.  The espree parser _does_ provide visitor keys as a  prop on the parser itself.  However, this logic was only returning it when it found that "espree" was somewhere in the `parserPath` (which doesn't exist on flat config).

I changed it so that it first does the check for the old-school babel parser using parser path, and then just checks the parser instance for the presence of `VisitorKeys`, rather than using parserPath at all. The reason for the re-order is that if the old babel-eslint parser, for some reason has `VisitorKeys`, having the new condition first, would change the behavior.

Author: michaelfaith

utils/parse.js

@@ -29,12 +29,18 @@ function keysFromParser(parserPath, parserInstance, parsedResult) {
   if (parsedResult && parsedResult.visitorKeys) {
     return parsedResult.visitorKeys;
   }
-  if (typeof parserPath === 'string' && (/.*espree.*/).test(parserPath)) {
-    return parserInstance.VisitorKeys;
-  }
+  // The old babel parser doesn't have a `parseForESLint` eslint function, so we don't end
+  // up with a `parsedResult` here.  It also doesn't expose the visitor keys on the parser itself,
+  // so we have to try and infer the visitor-keys module from the parserPath.
+  // This is NOT supported in flat config!
   if (typeof parserPath === 'string' && (/.*babel-eslint.*/).test(parserPath)) {
     return getBabelEslintVisitorKeys(parserPath);
   }
+  // The espree parser doesn't have the `parseForESLint` function, so we don't ended up with a
+  // `parsedResult` here, but it does expose the visitor keys on the parser instance that we can use.
+  if (parserInstance && parserInstance.VisitorKeys) {
+    return parserInstance.VisitorKeys;
+  }
   return null;
 }