fix(perf): Improve getSelector performance

Author: dylanb

lib/core/base/audit.js

@@ -180,7 +180,6 @@ Audit.prototype.run = function (context, options, resolve, reject) {
 		}
 	});
 	q.then(function (results) {
-		axe._tree = undefined; // empty the tree
 		axe._selectCache = undefined; // remove the cache
 		resolve(results.filter(function (result) { return !!result; }));
 	}).catch(reject);

lib/core/public/run.js

@@ -127,6 +127,8 @@ axe.run = function (context, options, callback) {
 		try {
 			let reporter = axe.getReporter(options.reporter);
 			let results = reporter(rawResults, options, respond);
+			axe._selectorData = undefined;
+			axe._tree = undefined;
 			if (results !== undefined) {
 				respond(results);
 			}

lib/core/utils/collect-results-from-frames.js

@@ -1,7 +1,11 @@
 
 function err(message, node) {
 	'use strict';
-	return new Error(message + ': ' + axe.utils.getSelector(node));
+	var selector;
+	if (axe._tree) {
+		selector = axe.utils.getSelector(node);
+	}
+	return new Error(message + ': ' + (selector || node));
 }
 
 /**