[ci] Enforce a minimum Kotlin version in examples (#3979)

https://github.com/flutter/packages/pull/3973 caused an out-of-band
failure after publishing, because an example that uses `url_launcher`
had a too-old Kotlin version set. This is not something we consider
client-breaking because `flutter` prodives a very clear error message
with a straightforward and actionable fix step (update the app's Kotlin
version), so the fix for the breakage is just to update our own
examples.

Since increasingly we're likely to hit problems where modern version of
dependencies don't work with old version of Kotlin, this adds repo-wide
CI enforcement that examples are set to a minimum version (matching the
current `flutter/flutter` template; we can increase this over time as we
feel it's useful to do so).

script/tool/test/gradle_check_command_test.dart

@@ -123,6 +123,7 @@ dependencies {
     RepositoryPackage package, {
     required String pluginName,
     required bool warningsConfigured,
+    String? kotlinVersion,
   }) {
     final File buildGradle = package
         .platformDirectory(FlutterPlatform.android)
@@ -140,6 +141,7 @@ gradle.projectsEvaluated {
 ''';
     buildGradle.writeAsStringSync('''
 buildscript {
+    ${kotlinVersion == null ? '' : "ext.kotlin_version = '$kotlinVersion'"}
     repositories {
         google()
         mavenCentral()
@@ -228,9 +230,12 @@ dependencies {
     bool includeNamespace = true,
     bool commentNamespace = false,
     bool warningsConfigured = true,
+    String? kotlinVersion,
   }) {
     writeFakeExampleTopLevelBuildGradle(package,
-        pluginName: pluginName, warningsConfigured: warningsConfigured);
+        pluginName: pluginName,
+        warningsConfigured: warningsConfigured,
+        kotlinVersion: kotlinVersion);
     writeFakeExampleAppBuildGradle(package,
         includeNamespace: includeNamespace, commentNamespace: commentNamespace);
   }
@@ -644,4 +649,99 @@ dependencies {
           ],
         ));
   });
+
+  group('Kotlin version check', () {
+    test('passes if not set', () async {
+      const String packageName = 'a_package';
+      final RepositoryPackage package =
+          createFakePackage('a_package', packagesDir);
+      writeFakePluginBuildGradle(package, includeLanguageVersion: true);
+      writeFakeManifest(package);
+      final RepositoryPackage example = package.getExamples().first;
+      writeFakeExampleBuildGradles(example, pluginName: packageName);
+      writeFakeManifest(example, isApp: true);
+
+      final List<String> output =
+          await runCapturingPrint(runner, <String>['gradle-check']);
+
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Validating android/build.gradle'),
+        ]),
+      );
+    });
+
+    test('passes if at the minimum allowed version', () async {
+      const String packageName = 'a_package';
+      final RepositoryPackage package =
+          createFakePackage('a_package', packagesDir);
+      writeFakePluginBuildGradle(package, includeLanguageVersion: true);
+      writeFakeManifest(package);
+      final RepositoryPackage example = package.getExamples().first;
+      writeFakeExampleBuildGradles(example,
+          pluginName: packageName, kotlinVersion: minKotlinVersion.toString());
+      writeFakeManifest(example, isApp: true);
+
+      final List<String> output =
+          await runCapturingPrint(runner, <String>['gradle-check']);
+
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Validating android/build.gradle'),
+        ]),
+      );
+    });
+
+    test('passes if above the minimum allowed version', () async {
+      const String packageName = 'a_package';
+      final RepositoryPackage package =
+          createFakePackage('a_package', packagesDir);
+      writeFakePluginBuildGradle(package, includeLanguageVersion: true);
+      writeFakeManifest(package);
+      final RepositoryPackage example = package.getExamples().first;
+      writeFakeExampleBuildGradles(example,
+          pluginName: packageName, kotlinVersion: '99.99.0');
+      writeFakeManifest(example, isApp: true);
+
+      final List<String> output =
+          await runCapturingPrint(runner, <String>['gradle-check']);
+
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Validating android/build.gradle'),
+        ]),
+      );
+    });
+
+    test('fails if below the minimum allowed version', () async {
+      const String packageName = 'a_package';
+      final RepositoryPackage package =
+          createFakePackage('a_package', packagesDir);
+      writeFakePluginBuildGradle(package, includeLanguageVersion: true);
+      writeFakeManifest(package);
+      final RepositoryPackage example = package.getExamples().first;
+      writeFakeExampleBuildGradles(example,
+          pluginName: packageName, kotlinVersion: '1.6.21');
+      writeFakeManifest(example, isApp: true);
+
+      Error? commandError;
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['gradle-check'], errorHandler: (Error e) {
+        commandError = e;
+      });
+
+      expect(commandError, isA<ToolExit>());
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('build.gradle sets "ext.kotlin_version" to "1.6.21". The '
+              'minimum Kotlin version that can be specified is '
+              '$minKotlinVersion, for compatibility with modern dependencies.'),
+        ]),
+      );
+    });
+  });
 }