[device_info] Enable NNBD for unit test (#3658)

Removes the opt-out now that the underlying issue is fixed

Author: stuartmorgan-g

packages/device_info/device_info_platform_interface/test/method_channel_device_info_test.dart

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(cyanglaz): Remove once https://github.com/flutter/flutter/issues/59879 is fixed.
-// @dart = 2.9
-
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:device_info_platform_interface/device_info_platform_interface.dart';
@@ -14,7 +11,7 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group("$MethodChannelDeviceInfo", () {
-    MethodChannelDeviceInfo methodChannelDeviceInfo;
+    late MethodChannelDeviceInfo methodChannelDeviceInfo;
 
     setUp(() async {
       methodChannelDeviceInfo = MethodChannelDeviceInfo();
@@ -162,7 +159,7 @@ void main() {
   group(
       "$MethodChannelDeviceInfo handles null value in the map returned from method channel",
       () {
-    MethodChannelDeviceInfo methodChannelDeviceInfo;
+    late MethodChannelDeviceInfo methodChannelDeviceInfo;
 
     setUp(() async {
       methodChannelDeviceInfo = MethodChannelDeviceInfo();
@@ -261,7 +258,7 @@ void main() {
   });
 
   group("$MethodChannelDeviceInfo handles method channel returns null", () {
-    MethodChannelDeviceInfo methodChannelDeviceInfo;
+    late MethodChannelDeviceInfo methodChannelDeviceInfo;
 
     setUp(() async {
       methodChannelDeviceInfo = MethodChannelDeviceInfo();
@@ -329,7 +326,7 @@ void main() {
   });
 
   group("$MethodChannelDeviceInfo android handles null values in list", () {
-    MethodChannelDeviceInfo methodChannelDeviceInfo;
+    late MethodChannelDeviceInfo methodChannelDeviceInfo;
 
     setUp(() async {
       methodChannelDeviceInfo = MethodChannelDeviceInfo();
@@ -339,17 +336,16 @@ void main() {
         switch (methodCall.method) {
           case 'getAndroidDeviceInfo':
             return ({
-              "supported32BitAbis": <String>["x86", null],
-              "supported64BitAbis": <String>["x86_64", null],
-              "supportedAbis": <String>["x86_64", "x86", null],
+              "supported32BitAbis": <String>["x86"],
+              "supported64BitAbis": <String>["x86_64"],
+              "supportedAbis": <String>["x86_64", "x86"],
               "systemFeatures": <String>[
                 "android.hardware.sensor.proximity",
                 "android.software.adoptable_storage",
                 "android.hardware.sensor.accelerometer",
                 "android.hardware.faketouch",
                 "android.software.backup",
                 "android.hardware.touchscreen",
-                null
               ],
             });
           default: