Exclude non Android plugins from Gradle build (#40640)

Before this change, having an Android app depend on a plugin that has no android implementation resulted in a Gradle build failure.

This scenario is likely to become more common if we're enabling federated plugins, as the package implementing just the desktop implementation of a plugin won't have an Android implementation.

This changes the Gradle plugin to not try to build any plugins that doesn't have an android/build.gradle file.

dev/bots/test.dart

@@ -1021,6 +1021,7 @@ Future<void> _androidGradleTests(String subShard) async {
     await _runDevicelabTest('gradle_plugin_light_apk_test', env: env);
     await _runDevicelabTest('gradle_plugin_fat_apk_test', env: env);
     await _runDevicelabTest('gradle_r8_test', env: env);
+    await _runDevicelabTest('gradle_non_android_plugin_test', env: env);
   }
   if (subShard == 'gradle2') {
     await _runDevicelabTest('gradle_plugin_bundle_test', env: env);

dev/devicelab/bin/tasks/gradle_non_android_plugin_test.dart

@@ -0,0 +1,107 @@
+// Copyright (c) 2019 The Chromium 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 'dart:async';
+import 'dart:io';
+
+import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/framework/utils.dart';
+import 'package:path/path.dart' as path;
+
+/// Tests that a flutter app that depends on a non-Android plugin
+/// (an iOS only plugin in this case) can still build for Android.
+Future<void> main() async {
+  await task(() async {
+
+    section('Find Java');
+
+    final String javaHome = await findJavaHome();
+    if (javaHome == null)
+      return TaskResult.failure('Could not find Java');
+    print('\nUsing JAVA_HOME=$javaHome');
+
+    section('Create Flutter plugin project');
+
+    final Directory tempDir = Directory.systemTemp.createTempSync('flutter_plugin_test.');
+    final Directory projectDir = Directory(path.join(tempDir.path, 'ios_only'));
+    try {
+      await inDirectory(tempDir, () async {
+        await flutter(
+          'create',
+          options: <String>[
+            '--org', 'io.flutter.devicelab',
+            '-t', 'plugin',
+            'ios_only',
+          ],
+        );
+      });
+
+      section('Delete plugin\'s Android folder');
+
+      final File androidFolder = File(path.join(
+        projectDir.path,
+        'android',
+      ));
+      androidFolder.deleteSync(recursive: true);
+
+      section('Update pubspec.yaml to iOS only plugin');
+      final File pubspecFile = File(path.join(projectDir.path, 'pubspec.yaml'));
+      final String pubspecString = pubspecFile.readAsStringSync();
+
+      final StringBuffer iosOnlyPubspec = StringBuffer();
+      for (String line in pubspecString.split('\n')) {
+        if (line.startsWith('    androidPackage:')) {
+          continue;
+        }
+        if (line.startsWith('    pluginClass:')) {
+          iosOnlyPubspec.write('    platforms:\n');
+          iosOnlyPubspec.write('      ios:\n');
+          iosOnlyPubspec.write('        pluginClass: IosOnlyPlugin\n');
+          continue;
+        }
+        iosOnlyPubspec.write('$line\n');
+      }
+
+      pubspecFile.writeAsStringSync(iosOnlyPubspec.toString());
+
+      section('Build example APK');
+
+      final StringBuffer stderr = StringBuffer();
+
+      final Directory exampleDir = Directory(path.join(projectDir.path, 'example'));
+      await inDirectory(exampleDir, () async {
+        await evalFlutter(
+          'build',
+          options: <String>[
+            'apk',
+            '--target-platform', 'android-arm',
+            '--verbose',
+          ],
+          canFail: true,
+          stderr: stderr,
+        );
+      });
+
+      section('Check that the example APK was built');
+
+      final String exampleAppApk = path.join(
+        exampleDir.path,
+        'build',
+        'app',
+        'outputs',
+        'apk',
+        'release',
+        'app-release.apk',
+      );
+      if (!exists(File(exampleAppApk))) {
+        return TaskResult.failure('Failed to build example app');
+      }
+      return TaskResult.success(null);
+    } catch (e) {
+      return TaskResult.failure(e.toString());
+    } finally {
+      rmTree(tempDir);
+    }
+  });
+}

packages/flutter_tools/gradle/flutter.gradle

@@ -329,7 +329,18 @@ class FlutterPlugin implements Plugin<Project> {
 
     private Properties getPluginList() {
         File pluginsFile = new File(project.projectDir.parentFile.parentFile, '.flutter-plugins')
-        return readPropertiesIfExist(pluginsFile)
+        Properties allPlugins = readPropertiesIfExist(pluginsFile)
+        Properties androidPlugins = new Properties()
+        allPlugins.each { name, path ->
+            File editableAndroidProject = new File(path, 'android' + File.separator + 'build.gradle')
+            if (editableAndroidProject.exists()) {
+                androidPlugins.setProperty(name, path)
+            }
+            // TODO(amirh): log an error if this plugin was specified to be an Android
+            // plugin according to the new schema, and was missing a build.gradle file.
+            // https://github.com/flutter/flutter/issues/40784
+        }
+        return androidPlugins
     }
 
     private void addPluginTasks() {