Use CLIEngine#getRules when available

Rule gathering from the ESLint instance has been expanded to take
advantage of the new `CLIEngine#getRules` method if it is available.

spec/worker-helpers-spec.js

@@ -213,6 +213,13 @@ describe('Worker Helpers', () => {
   })
 
   describe('getRules', () => {
+    it('works with the getRules function introduced in ESLint v4.15.0', () => {
+      const cliEngine = {
+        getRules: () => 'foo'
+      }
+      expect(Helpers.getRules(cliEngine)).toBe('foo')
+    })
+
     it('works with the hidden linter in ESLint v4 before v4.15.0', () => {
       const cliEngine = {
         linter: {

src/worker-helpers.js

@@ -192,6 +192,12 @@ export function getCLIEngineOptions(type, config, rules, filePath, fileDir, give
  *                            properties as the contents.
  */
 export function getRules(cliEngine) {
+  // Pull the list of rules used directly from the CLIEngine
+  // Added in https://github.com/eslint/eslint/pull/9782
+  if (Object.prototype.hasOwnProperty.call(cliEngine, 'getRules')) {
+    return cliEngine.getRules()
+  }
+
   // Attempt to use the internal (undocumented) `linter` instance attached to
   // the CLIEngine to get the loaded rules (including plugin rules).
   // Added in ESLint v4