diff --git a/packages/flutter_tools/lib/src/resident_runner.dart b/packages/flutter_tools/lib/src/resident_runner.dart
index 27fa9b1c444..9e731aa5bf1 100644
--- a/packages/flutter_tools/lib/src/resident_runner.dart
+++ b/packages/flutter_tools/lib/src/resident_runner.dart
@@ -83,10 +83,8 @@ class FlutterDevice {
     required Platform platform,
     TargetModel targetModel = TargetModel.flutter,
     List<String>? experimentalFlags,
-    ResidentCompiler? generator,
     String? userIdentifier,
   }) async {
-    ResidentCompiler generator;
     final TargetPlatform targetPlatform = await device.targetPlatform;
     if (device.platformType == PlatformType.fuchsia) {
       targetModel = TargetModel.flutterRunner;
@@ -111,6 +109,8 @@ class FlutterDevice {
       fileSystem: globals.fs,
     );
 
+    final ResidentCompiler generator;
+
     // For both web and non-web platforms we initialize dill to/from
     // a shared location for faster bootstrapping. If the compiler fails
     // due to a kernel target or version mismatch, no error is reported
@@ -119,7 +119,7 @@ class FlutterDevice {
     // used to file a bug, but the compiler will still start up correctly.
     if (targetPlatform == TargetPlatform.web_javascript) {
       // TODO(zanderso): consistently provide these flags across platforms.
-      late String platformDillName;
+      final String platformDillName;
       final List<String> extraFrontEndOptions = List<String>.of(buildInfo.extraFrontEndOptions);
       if (buildInfo.nullSafetyMode == NullSafetyMode.unsound) {
         platformDillName = 'ddc_outline.dill';
@@ -132,12 +132,12 @@ class FlutterDevice {
           extraFrontEndOptions.add('--sound-null-safety');
         }
       } else {
-        assert(false);
+        throw StateError('Expected buildInfo.nullSafetyMode to be one of unsound or sound, got ${buildInfo.nullSafetyMode}');
       }
 
       final String platformDillPath = globals.fs.path.join(
         getWebPlatformBinariesDirectory(globals.artifacts!, buildInfo.webRenderer).path,
-        platformDillName
+        platformDillName,
       );
 
       generator = ResidentCompiler(
diff --git a/packages/flutter_tools/lib/src/runner/flutter_command.dart b/packages/flutter_tools/lib/src/runner/flutter_command.dart
index 632baa289ba..d0a963c94f3 100644
--- a/packages/flutter_tools/lib/src/runner/flutter_command.dart
+++ b/packages/flutter_tools/lib/src/runner/flutter_command.dart
@@ -1093,7 +1093,11 @@ abstract class FlutterCommand extends Command<void> {
             (languageVersion.major == nullSafeVersion.major && languageVersion.minor >= nullSafeVersion.minor)) {
           nullSafetyMode = NullSafetyMode.sound;
         } else {
-          nullSafetyMode = NullSafetyMode.unsound;
+          throwToolExit(
+            'This application does not support sound null-safety (its language version is $languageVersion).\n'
+            'To build this application, you must provide the CLI flag --no-sound-null-safety. Dart 3 will only '
+            'support sound null safety, see https://dart.dev/null-safety.',
+          );
         }
       } else if (!wasNullSafetyFlagParsed) {
         // This mode is only used for commands which do not build a single target like
diff --git a/packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart b/packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart
index 779a31a9b75..4b3b729cb2c 100644
--- a/packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart
+++ b/packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart
@@ -554,6 +554,27 @@ void main() {
       ProcessManager: () => processManager,
     });
 
+    testUsingContext('tool exits on non-sound-null-safe code when explicit flag not passed', () async {
+      final DummyFlutterCommand flutterCommand = DummyFlutterCommand(packagesPath: 'foo');
+      flutterCommand.argParser
+          ..addFlag(FlutterOptions.kNullSafety, defaultsTo: true)
+          ..addOption('target');
+      final File targetFile = fileSystem.file('targetFile.dart')
+          ..writeAsStringSync('// @dart = 2.11');
+      expect(
+        () async => flutterCommand.getBuildInfo(
+          forcedBuildMode: BuildMode.debug,
+          forcedTargetFile: targetFile,
+        ),
+        throwsToolExit(
+          message: 'This application does not support sound null-safety (its language version is 2.11)',
+        ),
+      );
+    }, overrides: <Type, Generator>{
+      FileSystem: () => fileSystem,
+      ProcessManager: () => processManager,
+    });
+
     testUsingContext('use packagesPath to generate BuildInfo', () async {
       final DummyFlutterCommand flutterCommand = DummyFlutterCommand(packagesPath: 'foo');
       final BuildInfo buildInfo = await flutterCommand.getBuildInfo(forcedBuildMode: BuildMode.debug);
diff --git a/packages/flutter_tools/test/integration.shard/test_data/stepping_project.dart b/packages/flutter_tools/test/integration.shard/test_data/stepping_project.dart
index ce51fe469f1..902ce7974e7 100644
--- a/packages/flutter_tools/test/integration.shard/test_data/stepping_project.dart
+++ b/packages/flutter_tools/test/integration.shard/test_data/stepping_project.dart
@@ -65,7 +65,7 @@ class WebSteppingProject extends Project {
   final String pubspec = '''
   name: test
   environment:
-    sdk: '>=2.10.0 <4.0.0'
+    sdk: '>=2.12.0 <4.0.0'
   dependencies:
     flutter:
       sdk: flutter