Migrate create command to null safety (#104484)

packages/flutter_tools/lib/src/template.dart

@@ -155,7 +155,7 @@ class Template {
   /// May throw a [ToolExit] if the directory is not writable.
   int render(
     Directory destination,
-    Map<String, Object> context, {
+    Map<String, Object?> context, {
     bool overwriteExisting = true,
     bool printStatusWhenWriting = true,
   }) {

packages/flutter_tools/test/src/test_flutter_command_runner.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:async';
 
 import 'package:args/args.dart';
@@ -20,7 +18,7 @@ import 'package:flutter_tools/src/runner/flutter_command_runner.dart';
 
 export 'package:test_api/test_api.dart' hide test, isInstanceOf; // ignore: deprecated_member_use
 
-CommandRunner<void> createTestCommandRunner([ FlutterCommand command ]) {
+CommandRunner<void> createTestCommandRunner([ FlutterCommand? command ]) {
   final FlutterCommandRunner runner = TestFlutterCommandRunner();
   if (command != null) {
     runner.addCommand(command);
@@ -31,7 +29,7 @@ CommandRunner<void> createTestCommandRunner([ FlutterCommand command ]) {
 /// Creates a flutter project in the [temp] directory using the
 /// [arguments] list if specified, or `--no-pub` if not.
 /// Returns the path to the flutter project.
-Future<String> createProject(Directory temp, { List<String> arguments }) async {
+Future<String> createProject(Directory temp, { List<String>? arguments }) async {
   arguments ??= <String>['--no-pub'];
   final String projectPath = globals.fs.path.join(temp.path, 'flutter_project');
   final CreateCommand command = CreateCommand();
@@ -61,7 +59,7 @@ class TestFlutterCommandRunner extends FlutterCommandRunner {
           userMessages: UserMessages(),
         );
         // For compatibility with tests that set this to a relative path.
-        Cache.flutterRoot = globals.fs.path.normalize(globals.fs.path.absolute(Cache.flutterRoot));
+        Cache.flutterRoot = globals.fs.path.normalize(globals.fs.path.absolute(Cache.flutterRoot!));
         return super.runCommand(topLevelResults);
       }
     );