diff --git a/lighthouse-core/gather/driver.js b/lighthouse-core/gather/driver.js
index bb80ba984dc5..d33e73ec1501 100644
--- a/lighthouse-core/gather/driver.js
+++ b/lighthouse-core/gather/driver.js
@@ -947,7 +947,7 @@ class Driver {
     const milestone = (await this.getBrowserVersion()).milestone;
     if (milestone < 71) {
       const toplevelIndex = traceCategories.indexOf('disabled-by-default-lighthouse');
-      traceCategories.splice(toplevelIndex, 1, 'toplevel');
+      traceCategories[toplevelIndex] = 'toplevel';
     }
 
     const uniqueCategories = Array.from(new Set(traceCategories));
diff --git a/lighthouse-core/test/create-test-trace.js b/lighthouse-core/test/create-test-trace.js
index ccc5cd19e9fa..ece800f48cfd 100644
--- a/lighthouse-core/test/create-test-trace.js
+++ b/lighthouse-core/test/create-test-trace.js
@@ -16,13 +16,13 @@ const frame = '3EFC2700D7BC3F4734CAF2F726EFB78C';
  */
 function getTopLevelTask({ts, duration}) {
   return {
-    name: 'ThreadControllerImpl::RunTask',
+    name: 'RunTask',
     ts: ts * 1000,
     dur: duration * 1000,
     pid,
     tid,
     ph: 'X',
-    cat: 'toplevel',
+    cat: 'disabled-by-default-lighthouse',
     args: {
       src_file: '../../third_party/blink/renderer/core/fake_runner.cc',
       src_func: 'FakeRunnerFinished',
diff --git a/lighthouse-core/test/gather/driver-test.js b/lighthouse-core/test/gather/driver-test.js
index a8af8272f31c..dbbefe448e9d 100644
--- a/lighthouse-core/test/gather/driver-test.js
+++ b/lighthouse-core/test/gather/driver-test.js
@@ -6,7 +6,7 @@
 'use strict';
 
 let sendCommandParams = [];
-let sendCommandMockResponses = {};
+const sendCommandMockResponses = new Map();
 
 const Driver = require('../../gather/driver.js');
 const Connection = require('../../gather/connections/connection.js');
@@ -49,16 +49,16 @@ function createActiveWorker(id, url, controlledClients, status = 'activated') {
 }
 
 function createOnceMethodResponse(method, response) {
-  assert.deepEqual(!!sendCommandMockResponses[method], false, 'stub response already defined');
-  sendCommandMockResponses[method] = response;
+  assert.equal(sendCommandMockResponses.has(method), false, 'stub response already defined');
+  sendCommandMockResponses.set(method, response);
 }
 
 connection.sendCommand = function(command, params) {
   sendCommandParams.push({command, params});
 
-  const mockResponse = sendCommandMockResponses[command];
-  if (mockResponse) {
-    delete sendCommandMockResponses[command];
+  if (sendCommandMockResponses.has(command)) {
+    const mockResponse = sendCommandMockResponses.get(command);
+    sendCommandMockResponses.delete(command);
     return Promise.resolve(mockResponse);
   }
 
@@ -115,7 +115,7 @@ connection.sendCommand = function(command, params) {
 describe('Browser Driver', () => {
   beforeEach(() => {
     sendCommandParams = [];
-    sendCommandMockResponses = {};
+    sendCommandMockResponses.clear();
   });
 
   it('returns null when DOM.querySelector finds no node', () => {