fix: init fake runtime.lastError to null.

test/test-async-functions.js

@@ -10,7 +10,7 @@ describe("browser-polyfill", () => {
     it("returns a promise which resolves to the callback parameters", () => {
       const fakeChrome = {
         alarms: {clear: sinon.stub()},
-        runtime: {lastError: undefined},
+        runtime: {lastError: null},
       };
       return setupTestDOMWindow(fakeChrome).then(window => {
         fakeChrome.alarms.clear

test/test-runtime-onMessage.js

@@ -12,7 +12,7 @@ describe("browser-polyfill", () => {
 
       const fakeChrome = {
         runtime: {
-          lastError: undefined,
+          lastError: null,
           onMessage: {
             addListener: sinon.spy(),
             hasListener: sinon.stub(),
@@ -55,7 +55,7 @@ describe("browser-polyfill", () => {
     it("sends the returned value as a message response", () => {
       const fakeChrome = {
         runtime: {
-          lastError: undefined,
+          lastError: null,
           onMessage: {
             addListener: sinon.spy(),
           },