Allow Android to be missing from multi-platform manifest (#50304)

packages/flutter_tools/lib/src/flutter_manifest.dart

@@ -161,7 +161,11 @@ class FlutterManifest {
     if (isPlugin) {
       final YamlMap plugin = _flutterDescriptor['plugin'] as YamlMap;
       if (plugin.containsKey('platforms')) {
-        return plugin['platforms']['android']['package'] as String;
+        final YamlMap platforms = plugin['platforms'] as YamlMap;
+
+        if (platforms.containsKey('android')) {
+          return platforms['android']['package'] as String;
+        }
       } else {
         return plugin['androidPackage'] as String;
       }

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

@@ -387,7 +387,8 @@ flutter:
       expect(flutterManifest.isPlugin, true);
       expect(flutterManifest.androidPackage, 'com.example');
     });
-    test('allows a multi-plat plugin declaration', () async {
+
+    test('allows a multi-plat plugin declaration with android only', () async {
       const String manifest = '''
 name: test
 flutter:
@@ -402,6 +403,20 @@ flutter:
       expect(flutterManifest.androidPackage, 'com.example');
     });
 
+    test('allows a multi-plat plugin declaration with ios only', () async {
+      const String manifest = '''
+name: test
+flutter:
+    plugin:
+      platforms:
+        ios:
+          pluginClass: HelloPlugin
+''';
+      final FlutterManifest flutterManifest = FlutterManifest.createFromString(manifest);
+      expect(flutterManifest.isPlugin, true);
+      expect(flutterManifest.androidPackage, isNull);
+    });
+
     testUsingContext('handles an invalid plugin declaration', () async {
       const String manifest = '''
 name: test