Fix a bug to skip flutter_gen when synthetic packages are disabled (#160367)

Without this bug fix, when flipping the flag a lot of things blow up :)

(https://github.com/flutter/flutter/pull/160289 as an example of needing
this)

packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart

@@ -49,7 +49,7 @@ Future<void> generateLocalizationsSyntheticPackage({
     // Generate gen_l10n synthetic package only if synthetic-package: true or
     // synthetic-package is null.
     final bool? isSyntheticL10nPackage = value as bool?;
-    if (isSyntheticL10nPackage == false) {
+    if (isSyntheticL10nPackage == false || isSyntheticL10nPackage == null && featureFlags.isExplicitPackageDependenciesEnabled) {
       return;
     }
   } else if (featureFlags.isExplicitPackageDependenciesEnabled) {

packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart

@@ -28,6 +28,14 @@ void main() {
     return TestFeatureFlags(isExplicitPackageDependenciesEnabled: false);
   }
 
+  // TODO(matanlurey): Remove after `explicit-package-dependencies` is enabled by default.
+  // See https://github.com/flutter/flutter/issues/160257 for details.
+  FeatureFlags enableExplicitPackageDependencies() {
+    return TestFeatureFlags(
+      isExplicitPackageDependenciesEnabled: true,
+    );
+  }
+
   testUsingContext('calls buildSystem.build with blank l10n.yaml file', () async {
     // Project directory setup for gen_l10n logic
     final MemoryFileSystem fileSystem = MemoryFileSystem.test();
@@ -394,6 +402,42 @@ void main() {
     );
   });
 
+  testUsingContext('synthetic-package omitted with explicit-package-dependencies is a NOP', () async {
+    // Project directory setup for gen_l10n logic
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+
+    // Add generate:true to pubspec.yaml.
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    final String content = pubspecFile.readAsStringSync().replaceFirst(
+      '\nflutter:\n',
+      '\nflutter:\n  generate: true\n',
+    );
+    pubspecFile.writeAsStringSync(content);
+
+    final BufferLogger mockBufferLogger = BufferLogger.test();
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      fileSystem: fileSystem,
+      logger: mockBufferLogger,
+      artifacts: Artifacts.test(),
+      processManager: FakeProcessManager.empty(),
+    );
+    final TestBuildSystem buildSystem = TestBuildSystem.all(BuildResult(success: true));
+
+    await generateLocalizationsSyntheticPackage(
+      environment: environment,
+      buildSystem: buildSystem,
+      buildTargets: const NoOpBuildTargets(),
+    );
+
+    expect(
+      mockBufferLogger.warningText,
+      isNot(contains('https://flutter.dev/to/flutter-gen-deprecation')),
+    );
+  }, overrides: <Type, Generator> {
+    FeatureFlags: enableExplicitPackageDependencies,
+  });
+
   testUsingContext('synthetic-package: true with explicit-packages-resolution is an error', () async {
     // Project directory setup for gen_l10n logic
     final MemoryFileSystem fileSystem = MemoryFileSystem.test();