Set default flutter source directory for gradle builds (#142934)

See [#142498](https://github.com/flutter/flutter/pull/142498#discussion_r1478602032)
See this discussion: https://discord.com/channels/608014603317936148/1186378330178601000

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -67,7 +67,7 @@ class FlutterExtension {
      * Specifies the relative directory to the Flutter project directory.
      * In an app project, this is ../.. since the app's Gradle build file is under android/app.
      */
-    String source
+    String source = "../.."
 
     /** Allows to override the target file. Otherwise, the target is lib/main.dart. */
     String target
@@ -834,7 +834,7 @@ class FlutterPlugin implements Plugin<Project> {
     }
 
     private Properties getPluginList() {
-        File pluginsFile = new File(project.projectDir.parentFile.parentFile, '.flutter-plugins')
+        File pluginsFile = new File(getFlutterSourceDirectory(), '.flutter-plugins')
         Properties allPlugins = readPropertiesIfExist(pluginsFile)
         Properties androidPlugins = new Properties()
         allPlugins.each { name, path ->
@@ -871,7 +871,7 @@ class FlutterPlugin implements Plugin<Project> {
         // This means, `plugin-a` depends on `plugin-b` and `plugin-c`.
         // `plugin-b` depends on `plugin-c`.
         // `plugin-c` doesn't depend on anything.
-        File pluginsDependencyFile = new File(project.projectDir.parentFile.parentFile, '.flutter-plugins-dependencies')
+        File pluginsDependencyFile = new File(getFlutterSourceDirectory(), '.flutter-plugins-dependencies')
         if (pluginsDependencyFile.exists()) {
             def object = new JsonSlurper().parseText(pluginsDependencyFile.text)
             assert(object instanceof Map)

packages/flutter_tools/test/integration.shard/android_gradle_flutter_source_path_test.dart

@@ -0,0 +1,70 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/cache.dart';
+
+import '../src/common.dart';
+import 'test_data/deferred_components_project.dart';
+import 'test_data/project.dart';
+import 'test_utils.dart';
+
+void main() {
+  late Directory tempDir;
+
+  setUp(() {
+    Cache.flutterRoot = getFlutterRoot();
+    tempDir =
+        createResolvedTempDirectorySync('flutter_gradle_source_path_test.');
+  });
+
+  tearDown(() async {
+    tryToDelete(tempDir);
+  });
+
+  test('gradle task builds without setting a source path in app/build.gradle',
+      () async {
+    final Project project = DeferredComponentsProject(
+      MissingFlutterSourcePathDeferredComponentsConfig(),
+    );
+    final String flutterBin = fileSystem.path.join(
+      getFlutterRoot(),
+      'bin',
+      'flutter',
+    );
+
+    final Directory exampleAppDir = tempDir.childDirectory('example');
+    await project.setUpIn(exampleAppDir);
+
+    // Run flutter build apk to build example project.
+    final ProcessResult buildApkResult = processManager.runSync(<String>[
+      flutterBin,
+      ...getLocalEngineArguments(),
+      'build',
+      'apk',
+      '--debug',
+    ], workingDirectory: exampleAppDir.path);
+
+    expect(buildApkResult, const ProcessResultMatcher());
+  });
+}
+
+class MissingFlutterSourcePathDeferredComponentsConfig
+    extends BasicDeferredComponentsConfig {
+  final String _flutterSourcePath = '''
+  flutter {
+      source '../..'
+  }
+''';
+
+  @override
+  String get appBuild {
+    if (!super.appBuild.contains(_flutterSourcePath)) {
+      throw Exception(
+          'Flutter source path not found in original configuration!');
+    }
+    return super.appBuild.replaceAll(_flutterSourcePath, '');
+  }
+}