Migrate deferred_components_prebuild_validator to null safety (#83854)

packages/flutter_tools/lib/src/android/deferred_components_prebuild_validator.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.
 
-// @dart = 2.8
-
-import 'package:meta/meta.dart';
 import 'package:xml/xml.dart';
 
 import '../base/deferred_component.dart';
@@ -34,12 +31,12 @@ class DeferredComponentsPrebuildValidator extends DeferredComponentsValidator {
   /// change. This defaults to true.
   DeferredComponentsPrebuildValidator(Directory projectDir, Logger logger, Platform platform, {
     bool exitOnFail = true,
-    String title,
-    Directory templatesDir,
+    String? title,
+    Directory? templatesDir,
   }) : _templatesDir = templatesDir,
        super(projectDir, logger, platform, exitOnFail: exitOnFail, title: title);
 
-  final Directory _templatesDir;
+  final Directory? _templatesDir;
 
   /// Checks if an android dynamic feature module exists for each deferred
   /// component.
@@ -69,17 +66,20 @@ class DeferredComponentsPrebuildValidator extends DeferredComponentsValidator {
       );
       if (!androidFiles.verifyFilesExist()) {
         // generate into temp directory
-        final Map<String, List<File>> results =
-          await androidFiles.generateFiles(
+        final Map<String, List<File>> results = await androidFiles.generateFiles(
           alternateAndroidDir: outputDir,
           clearAlternateOutputDir: true,
         );
-        for (final File file in results['outputs']) {
+        if (results.containsKey('outputs')) {
+          for (final File file in results['outputs']!) {
             generatedFiles.add(file.path);
             changesMade = true;
           }
-        outputs.addAll(results['outputs']);
-        inputs.addAll(results['inputs']);
+          outputs.addAll(results['outputs']!);
+        }
+        if (results.containsKey('inputs')) {
+          inputs.addAll(results['inputs']!);
+        }
       }
     }
     return !changesMade;
@@ -148,16 +148,16 @@ class DeferredComponentsPrebuildValidator extends DeferredComponentsValidator {
       // wrong string stored.
       for (final XmlElement resources in document.findAllElements('resources')) {
         for (final XmlElement element in resources.findElements('string')) {
-          final String name = element.getAttribute('name');
+          final String? name = element.getAttribute('name');
           if (requiredEntriesMap.containsKey(name)) {
             if (element.text != null && element.text != requiredEntriesMap[name]) {
-              element.innerText = requiredEntriesMap[name];
+              element.innerText = requiredEntriesMap[name]!;
               modified = true;
             }
             requiredEntriesMap.remove(name);
           }
         }
-        for (final String key in requiredEntriesMap.keys) {
+        requiredEntriesMap.forEach((String key, String value) {
           modified = true;
           final XmlElement newStringElement = XmlElement(
             XmlName.fromString('string'),
@@ -165,11 +165,11 @@ class DeferredComponentsPrebuildValidator extends DeferredComponentsValidator {
               XmlAttribute(XmlName.fromString('name'), key),
             ],
             <XmlNode>[
-              XmlText(requiredEntriesMap[key]),
+              XmlText(value),
             ],
           );
           resources.children.add(newStringElement);
-        }
+        });
         break;
       }
       if (modified) {
@@ -211,17 +211,17 @@ class DeferredComponentsPrebuildValidator extends DeferredComponentsValidator {
 // directory.
 class _DeferredComponentAndroidFiles {
   _DeferredComponentAndroidFiles({
-    @required this.name,
-    @required this.projectDir,
-    this.logger,
-    Directory templatesDir,
+    required this.name,
+    required this.projectDir,
+    required this.logger,
+    Directory? templatesDir,
   }) : _templatesDir = templatesDir;
 
   // The name of the deferred component.
   final String name;
   final Directory projectDir;
   final Logger logger;
-  final Directory _templatesDir;
+  final Directory? _templatesDir;
 
   Directory get androidDir => projectDir.childDirectory('android');
   Directory get componentDir => androidDir.childDirectory(name);
@@ -235,7 +235,7 @@ class _DeferredComponentAndroidFiles {
   }
 
   // Generates any missing basic files for the dynamic feature into a temporary directory.
-  Future<Map<String, List<File>>> generateFiles({Directory alternateAndroidDir, bool clearAlternateOutputDir = false}) async {
+  Future<Map<String, List<File>>> generateFiles({Directory? alternateAndroidDir, bool clearAlternateOutputDir = false}) async {
     final Directory outputDir = alternateAndroidDir?.childDirectory(name) ?? componentDir;
     if (clearAlternateOutputDir && alternateAndroidDir != null) {
       ErrorHandlingFileSystem.deleteIfExists(outputDir);
@@ -251,9 +251,10 @@ class _DeferredComponentAndroidFiles {
   // generates default build.gradle and AndroidManifest.xml for the deferred component.
   Future<List<File>> _setupComponentFiles(Directory outputDir) async {
     Template template;
-    if (_templatesDir != null) {
-      final Directory templateComponentDir = _templatesDir.childDirectory('module${globals.fs.path.separator}android${globals.fs.path.separator}deferred_component');
-      template = Template(templateComponentDir, templateComponentDir, _templatesDir,
+    final Directory? templatesDir = _templatesDir;
+    if (templatesDir != null) {
+      final Directory templateComponentDir = templatesDir.childDirectory('module${globals.fs.path.separator}android${globals.fs.path.separator}deferred_component');
+      template = Template(templateComponentDir, templateComponentDir, templatesDir,
         fileSystem: globals.fs,
         templateManifest: null,
         logger: logger,