Avoid setting driver to null in tearDown

In all these cases it shouldn't be necessary and it simplifies the
code. With Null Safety it'll allow us to use the `late` keyword instead
of making everything nullable, which is much more convenient.

PiperOrigin-RevId: 324579665
diff --git a/test/async_cookies_test.dart b/test/async_cookies_test.dart
index 28e7c78..f33245d 100644
--- a/test/async_cookies_test.dart
+++ b/test/async_cookies_test.dart
@@ -32,10 +32,7 @@
     });
 
     tearDown(() async {
-      if (driver != null) {
-        await driver.quit();
-      }
-      driver = null;
+      await driver.quit();
     });
 
     test('add simple cookie and get', () async {
diff --git a/test/async_timeouts_test.dart b/test/async_timeouts_test.dart
index bfbaae1..4e404f7 100644
--- a/test/async_timeouts_test.dart
+++ b/test/async_timeouts_test.dart
@@ -29,10 +29,7 @@
     });
 
     tearDown(() async {
-      if (driver != null) {
-        await driver.quit();
-      }
-      driver = null;
+      await driver.quit();
     });
 
     // TODO(DrMarcII): Figure out how to tell if timeouts are correctly set
diff --git a/test/async_window_test.dart b/test/async_window_test.dart
index df3a6c0..0b6b51f 100644
--- a/test/async_window_test.dart
+++ b/test/async_window_test.dart
@@ -33,10 +33,7 @@
     });
 
     tearDown(() async {
-      if (driver != null) {
-        await driver.quit();
-      }
-      driver = null;
+      await driver.quit();
     });
 
     test('size', () async {
diff --git a/test/sync/basic_sync.dart b/test/sync/basic_sync.dart
index bd43621..17b5789 100644
--- a/test/sync/basic_sync.dart
+++ b/test/sync/basic_sync.dart
@@ -28,10 +28,7 @@
     });
 
     tearDown(() {
-      if (driver != null) {
-        driver.quit();
-      }
-      driver = null;
+      driver.quit();
     });
 
     test('can do basic post', () {
diff --git a/test/sync/command_event.dart b/test/sync/command_event.dart
index 96dce97..9dda010 100644
--- a/test/sync/command_event.dart
+++ b/test/sync/command_event.dart
@@ -40,7 +40,6 @@
     tearDown(() async {
       driver.quit();
       events.clear();
-      driver = null;
 
       await server.close(force: true);
     });
diff --git a/test/sync/cookies.dart b/test/sync/cookies.dart
index c885342..022482d 100644
--- a/test/sync/cookies.dart
+++ b/test/sync/cookies.dart
@@ -32,10 +32,7 @@
     });
 
     tearDown(() {
-      if (driver != null) {
-        driver.quit();
-      }
-      driver = null;
+      driver.quit();
     });
 
     test('add simple cookie and get', () {
diff --git a/test/sync/navigation.dart b/test/sync/navigation.dart
index 26159e1..7b89bb2 100644
--- a/test/sync/navigation.dart
+++ b/test/sync/navigation.dart
@@ -33,11 +33,7 @@
     });
 
     tearDown(() async {
-      if (driver != null) {
-        driver.quit();
-      }
-      driver = null;
-
+      driver.quit();
       await server.close(force: true);
     });
 
diff --git a/test/sync/timeouts.dart b/test/sync/timeouts.dart
index f69dedd..c3c3685 100644
--- a/test/sync/timeouts.dart
+++ b/test/sync/timeouts.dart
@@ -29,10 +29,7 @@
     });
 
     tearDown(() {
-      if (driver != null) {
-        driver.quit();
-      }
-      driver = null;
+      driver.quit();
     });
 
     // TODO(DrMarcII): Figure out how to tell if timeouts are correctly set
diff --git a/test/sync/window.dart b/test/sync/window.dart
index 33f3ced..d15f31f 100644
--- a/test/sync/window.dart
+++ b/test/sync/window.dart
@@ -32,10 +32,7 @@
     });
 
     tearDown(() {
-      if (driver != null) {
-        driver.quit();
-      }
-      driver = null;
+      driver.quit();
     });
 
     test('size', () {