[flutter_tools] Allow flutter build aar to know the null safety mode ahead of time (#80000)

packages/flutter_tools/lib/src/commands/build_aar.dart

@@ -3,12 +3,13 @@
 // found in the LICENSE file.
 
 // @dart = 2.8
-
 import 'package:meta/meta.dart';
 
 import '../android/android_builder.dart';
 import '../android/gradle_utils.dart';
 import '../base/common.dart';
+
+import '../base/file_system.dart';
 import '../base/os.dart';
 import '../build_info.dart';
 import '../cache.dart';
@@ -96,7 +97,9 @@ class BuildAarCommand extends BuildSubCommand {
       'By default, AARs are built for `release`, `debug` and `profile`.\n'
       'The POM file is used to include the dependencies that the AAR was compiled against.\n'
       'To learn more about how to use these artifacts, see '
-      'https://flutter.dev/go/build-aar';
+      'https://flutter.dev/go/build-aar\n'
+      'Note: this command builds applications assuming that the entrypoint is lib/main.dart. '
+      'This cannot currently be configured.';
 
   @override
   Future<FlutterCommandResult> runCommand() async {
@@ -114,11 +117,15 @@ class BuildAarCommand extends BuildSubCommand {
       ? stringArg('build-number')
       : '1.0';
 
+    final File targetFile = globals.fs.file(globals.fs.path.join('lib', 'main.dart'));
     for (final String buildMode in const <String>['debug', 'profile', 'release']) {
       if (boolArg(buildMode)) {
         androidBuildInfo.add(
           AndroidBuildInfo(
-            await getBuildInfo(forcedBuildMode: BuildMode.fromName(buildMode)),
+            await getBuildInfo(
+              forcedBuildMode: BuildMode.fromName(buildMode),
+              forcedTargetFile: targetFile,
+            ),
             targetArchs: targetArchitectures,
           )
         );
@@ -131,7 +138,7 @@ class BuildAarCommand extends BuildSubCommand {
     displayNullSafetyMode(androidBuildInfo.first.buildInfo);
     await androidBuilder.buildAar(
       project: _getProject(),
-      target: '', // Not needed because this command only builds Android's code.
+      target: targetFile.path,
       androidBuildInfo: androidBuildInfo,
       outputDirectoryPath: stringArg('output-dir'),
       buildNumber: buildNumber,

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -841,7 +841,7 @@ abstract class FlutterCommand extends Command<void> {
   ///
   /// Throws a [ToolExit] if the current set of options is not compatible with
   /// each other.
-  Future<BuildInfo> getBuildInfo({ BuildMode forcedBuildMode }) async {
+  Future<BuildInfo> getBuildInfo({ BuildMode forcedBuildMode, File forcedTargetFile }) async {
     final bool trackWidgetCreation = argParser.options.containsKey('track-widget-creation') &&
       boolArg('track-widget-creation');
 
@@ -891,8 +891,8 @@ abstract class FlutterCommand extends Command<void> {
       // passing a flag. Examine the entrypoint file to determine if it
       // is opted in or out.
       final bool wasNullSafetyFlagParsed = argResults.wasParsed(FlutterOptions.kNullSafety);
-      if (!wasNullSafetyFlagParsed && argParser.options.containsKey('target')) {
-        final File entrypointFile = globals.fs.file(targetFile);
+      if (!wasNullSafetyFlagParsed && (argParser.options.containsKey('target') || forcedTargetFile != null)) {
+        final File entrypointFile = forcedTargetFile ?? globals.fs.file(targetFile);
         final LanguageVersion languageVersion = determineLanguageVersion(
           entrypointFile,
           packageConfig.packageOf(entrypointFile.absolute.uri),

packages/flutter_tools/test/commands.shard/permeable/build_aar_test.dart

@@ -185,6 +185,7 @@ void main() {
       expect(buildInfo.splitDebugInfoPath, '/project-name/v1.2.3/');
       expect(buildInfo.dartObfuscation, isTrue);
       expect(buildInfo.dartDefines.contains('foo=bar'), isTrue);
+      expect(buildInfo.nullSafetyMode, NullSafetyMode.sound);
     }, overrides: <Type, Generator>{
       AndroidBuilder: () => fakeAndroidBuilder,
     });